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Research
Cite this article: delBarco-Trillo J, Garcı́aÁlvarez O, Soler AJ, Tourmente M, Garde JJ,
Roldan ERS. 2016 A cost for high levels of
sperm competition in rodents: increased sperm
DNA fragmentation. Proc. R. Soc. B 283:
20152708.
http://dx.doi.org/10.1098/rspb.2015.2708
Received: 13 November 2015
Accepted: 9 February 2016
Subject Areas:
cellular biology, evolution, physiology
Keywords:
sperm competition, rodents, sperm DNA
fragmentation, capacitation, oxidative stress,
sperm chromatin structure assay
A cost for high levels of sperm
competition in rodents: increased sperm
DNA fragmentation
Javier delBarco-Trillo1,3, Olga Garcı́a-Álvarez2, Ana Josefa Soler2,
Maximiliano Tourmente1, José Julián Garde2 and Eduardo R. S. Roldan1
1
Reproductive Ecology and Biology Group, Museo Nacional de Ciencias Naturales (CSIC), Madrid 28006, Spain
SaBio IREC (CSIC-UCLM-JCCM), Albacete 02071, Spain
3
School of Natural Sciences and Psychology, Liverpool John Moores University, Liverpool L3 3AF, UK
2
Jd-T, 0000-0002-9948-6674; OG-A, 0000-0003-4310-0557; AJS, 0000-0002-6429-5157;
MT, 0000-0002-5833-9117; JJG, 0000-0002-3667-6518
Sperm competition, a prevalent evolutionary process in which the spermatozoa
of two or more males compete for the fertilization of the same ovum, leads to
morphological and physiological adaptations, including increases in energetic
metabolism that may serve to propel sperm faster but that may have negative
effects on DNA integrity. Sperm DNA damage is associated with reduced rates
of fertilization, embryo and fetal loss, offspring mortality, and mutations leading to genetic disease. We tested whether high levels of sperm competition
affect sperm DNA integrity. We evaluated sperm DNA integrity in 18 species
of rodents that differ in their levels of sperm competition using the sperm chromatin structure assay. DNA integrity was assessed upon sperm collection, in
response to incubation under capacitating or non-capacitating conditions,
and after exposure to physical and chemical stressors. Sperm DNA was
very resistant to physical and chemical stressors, whereas incubation in noncapacitating and capacitating conditions resulted in only a small increase in
sperm DNA damage. Importantly, levels of sperm competition were positively
associated with sperm DNA fragmentation across rodent species. This is
the first evidence showing that high levels of sperm competition lead to an
important cost in the form of increased sperm DNA damage.
1. Introduction
Author for correspondence:
Javier delBarco-Trillo
e-mail: delbarcotrillo@gmail.com
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rspb.2015.2708 or
via http://rspb.royalsocietypublishing.org.
Sperm competition occurs when females mate with more than one male and the
spermatozoa of those males compete for the fertilization of the same ovum [1,2].
Sperm competition is a prevalent phenomenon and its occurrence leads to several
evolutionary adaptations at both the morphological and physiological levels [2,3].
High levels of sperm competition are associated with an increase in the production, storage and allocation of spermatozoa, as well as with enhanced sperm
function [3–5]. In rodents, high levels of sperm competition lead to a higher proportion of spermatozoa that are morphologically normal, motile and capable of
reaching and fertilizing the ovum [6,7], as well as in modifications in sperm dimensions that may result in improvements in sperm movement [8,9]. In many taxa,
sperm swimming velocity, an important feature of sperm function, is also
higher in those species that experience high levels of sperm competition [9–12].
Sperm DNA integrity is vital for the transmission of paternal genetic
material. The occurrence of DNA damage in sperm cells is associated with
reduced rates of fertilization, abortion and developmental problems with
adverse effects for offspring [13 –17]. The origins of this sperm DNA damage
are varied, including strand breaks that originate during the chromatin
remodelling during spermiogenesis and DNA fragmentation induced by oxidative stress [18,19]. Sperm cells are especially sensitive to DNA damage because
of the absence of DNA repair mechanisms; the process of abortive apoptosis, in
which sperm cells are only able to undergo a restricted apoptotic process that
leads to DNA fragmentation but renders these cells still capable of fertilization;
& 2016 The Author(s) Published by the Royal Society. All rights reserved.
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2. Material and methods
(a) Animals
We studied adult males (when ages were known, we used males
that were four to six months of age) from a total of 18 species.
Males from Mus musculus (n ¼ 16), M. pahari (n ¼ 4), M. spretus
(n ¼ 16) and M. spicilegus (n ¼ 15) came from wild-derived colonies, which had been kept in captivity at the Museo Nacional de
Ciencias Naturales for only a few generations. Males from
M. castaneus (n ¼ 8), M. caroli (n ¼ 10), M. domesticus (n ¼ 6)
and M. macedonicus (n ¼ 7) were purchased from the Laboratory
of Genome and Populations of the University of Montpellier II,
France. Males from Cricetulus griseus (n ¼ 5), Lemniscomys
barbarus (n ¼ 9), Mastomys natalensis (n ¼ 6), Mesocricetus auratus
(n ¼ 6), M. minutoides (n ¼ 8), Micromys minutus (n ¼ 5), Phodopus
campbelli (n ¼ 5), P. roborovskii (n ¼ 5) and P. sungorus (n ¼ 7)
were purchased from local vendors. Males from Apodemus
sylvaticus (n ¼ 13) were trapped in Palencia, Spain, during the
reproductive season, taken to the laboratory, and housed in individual cages for 10 – 15 days before sample collection (to
minimize perceived risk of sperm competition by males). All
males were maintained under standard conditions (14 h light–
10 h darkness, 22 – 248C, 55 – 60% relative humidity); with food
2
Proc. R. Soc. B 283: 20152708
production of ROS and thus increased sperm DNA damage.
To test this prediction, we incubated sperm cells from three
species of mice from the genus Mus that differ in their levels of
sperm competition [6] under non-capacitating and capacitating
conditions, and quantified sperm DNA integrity.
Third, we reasoned that spermatozoa from species with
varying levels of sperm competition may respond differently
to a series of external stressors. To assess the effect of different
stressors, we exposed sperm cells from the same three Mus
species to physical (freeze–thawing plus vortexing) and
chemical (DNase-I and H2O2) stressors to determine if any
increase in DNA damage promoted by such stressors is influenced by differing levels of sperm competition. DNase-I is an
endonuclease that cleaves DNA phosphodiester bonds and
thus induces DNA strand breaks. Exposing spermatozoa
in vitro to DNase-I is an indirect approach to determine if
species differ in their degree of sperm DNA compaction.
During sperm maturation, the chromatin becomes extremely
condensed when histones are replaced by protamines [26].
The inter- and intra-molecular disulfide bonds between the
protamine molecules confer chromatin compaction and stabilization [27], and thus higher protection for the nuclear DNA
from damaging factors, including ROS. In fact, deficient protamination of sperm chromatin results in DNA damage and
infertility [28]. Given that such DNA compaction may differ
between species, the spermatozoa of different species may
endure different levels of DNA damage under the same conditions [29,30]. Since the action of DNase-I depends on its
access to DNA, a higher induction of DNA strand breaks by
DNase-I in some species would give a measure of impaired
compaction in those species. H2O2 is normally used as an oxidative stressor due to its membrane permeability and readiness
to form the highly reactive hydroxyl radical that predominantly causes single-strand breaks and oxidative base
damage [31]. By exposing sperm cells to different levels of
H2O2 and assessing the ensuing DNA damage, we were able
to determine if the three Mus species have different sensitivity
to oxidative stress [14], and, if so, whether such species differences are related to their differing levels of sperm competition.
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and the low levels of ROS-scavenging enzymes due to the
limited volume of the cytosolic space in sperm cells [14,18].
Given that high levels of sperm competition are associated
with a general enhancement of sperm function, two general
scenarios may be possible. On the one hand, under high
sperm competition conditions, species may evolve spermatozoa and accessory gland fluids that manage to prevent sperm
DNA damage, and therefore it could be predicted that in
species with high levels of sperm competition, spermatozoa
will have (i) lower levels of baseline DNA damage and
(ii) higher resistance to stressors that can impact DNA integrity.
Such reduced baseline levels and higher protection against
DNA damage can be attained, for example, by increasing antioxidant measures in the male reproductive tract, seminal fluid
and spermatozoa themselves. On the other hand, the increased
metabolism needed to fuel higher sperm swimming velocity in
species that experience high levels of sperm competition [20]
could lead to a marked rise in the production of reactive
oxygen species (ROS) within sperm cells, resulting in a net
loss in DNA integrity, despite countermeasures evolved to prevent or repair such damage. In addition, the faster rates of
spermatogenesis in species with high levels of sperm competition [3] may result in less controlled DNA condensation
and packaging of DNA leading to higher incidences of DNA
fragmentation. In any of the latter two cases, an increase in
DNA damage should be regarded as a cost accrued by species
with high levels of sperm competition.
In order to test these two opposing predictions relating levels
of sperm competition, sperm structure and function, and sperm
DNA damage, we evaluated sperm DNA integrity in three
series of experiments using the sperm chromatin structure
assay (SCSA). The SCSA is an optimal technique to quantify
populations of DNA-fragmented spermatozoa in rodents [21]
and other mammals [22], including humans [23]. First, we performed a comparative analysis to assess baseline levels of DNA
fragmentation among 18 species of rodents that differ in their
levels of sperm competition. In addition, we used SCSA data to
determine if the proportion of spermatozoa in immature stages
(see ‘DNA fragmentation assessment’ below) is higher in species
with high levels of sperm competition, which could be related to
faster spermatogenesis in these species [3]. We also assessed the
possible changes in DNA integrity after a long-term incubation
that mimics the period of time and conditions of sperm survival
in the female tract before undergoing changes required for
fertilization. As already mentioned, spermatozoa are more
motile and swim faster in species with high levels of sperm
competition, possibly leading to an increased production of
ROS. We thus hypothesized that increases in sperm DNA
damage driven by incubation will be higher as the level of
sperm competition increases.
Second, we assessed whether capacitation of spermatozoa,
as influenced by different levels of sperm competition, has any
impact on sperm DNA damage. Capacitation is a physiological
switching-on that occurs in the oviduct and is essential for mammalian spermatozoa. Molecular and cellular changes taking
place during capacitation render spermatozoa capable of interacting with the ovum and surrounding coats, and undergoing
the acrosome reaction in response to ovum signals [24,25]. Completion of capacitation is accompanied by the development of
hyperactivated motility, which is thought to facilitate sperm
transport and ovum penetration during the final stages of fertilization [25]. Hyperactivated motility, because of its presumed
increase in energetic demands, could lead to an enhanced
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Table 1. Relationship between levels of sperm competition and SCSA values. Values represent the mean + s.e.m. RTS, relative testes size; tDFI, total DNA
fragmentation index; HDS, high DNA stainability.
n
body mass (g)
RTS
tDFI
HDS
Apodemus sylvaticus
Cricetulus griseus
13
5
29.85 + 0.69
33.72 + 0.38
0.96 + 0.02
1.78 + 0.04
2.27 + 0.05
3.83 + 0.11
19.11 + 2.06
27.65 + 2.58
39.34 + 2.95
15.03 + 2.36
Lemniscomys barbarus
Mastomys natalensis
9
6
42.45 + 1.36
80.76 + 5.52
0.7 + 0.06
0.92 + 0.06
1.25 + 0.1
1.02 + 0.06
14.27 + 2.49
27.66 + 3.16
13.51 + 3.77
37.27 + 3.66
Mesocricetus auratus
6
111.02 + 8.59
3.32 + 0.23
2.88 + 0.11
19 + 4.26
17.78 + 1.69
5
10
8.09 + 0.43
18.11 + 0.39
0.12 + 0.01
0.15 + 0.01
0.77 + 0.03
0.5 + 0.02
3.42 + 1.02
4.29 + 0.59
2.21 + 0.53
10.8 + 1.67
Mus castaneus
Mus domesticus
8
6
18.82 + 0.34
19.7 + 0.6
0.08 + 0.01
0.15 + 0.01
0.26 + 0.02
0.49 + 0.02
5.41 + 0.68
4.97 + 1.04
8.23 + 2.19
12.22 + 2.82
Mus macedonicus
Mus minutoides
7
8
20.1 + 0.64
5.84 + 0.48
0.3 + 0.01
0.11 + 0.01
0.95 + 0.03
0.89 + 0.06
3.92 + 1.29
7.18 + 0.2
8.74 + 0.81
3.51 + 0.28
Mus musculus
16
22. 86 + 1.29
0.14 + 0.01
0.42 + 0.02
3.55 + 1.09
9.35 + 0.67
Mus pahari
Mus spicilegus
4
15
33.15 + 0.61
17.85 + 0.56
0.13 + 0
0.45 + 0.01
0.28 + 0.01
1.58 + 0.03
4.49 + 0.53
3.42 + 0.59
9.04 + 1.82
10.28 + 0.76
Mus spretus
Phodopus campbelli
16
5
17.85 + 0.76
51.31 + 3.46
0.28 + 0.01
2.09 + 0.12
1 + 0.04
3.25 + 0.13
2.97 + 0.25
15.21 + 1.18
5.73 + 0.72
9.82 + 2.44
Phodopus roborovskii
5
24.64 + 1.06
0.93 + 0.09
2.53 + 0.21
17.2 + 3.17
5.05 + 0.98
Phodopus sungorus
7
43.08 + 1.87
0.9 + 0.05
1.6 + 0.08
Micromys minutus
Mus caroli
(rodent chow, Harlan Laboratories; seeds and fresh apple) and
water provided ad libitum.
(b) Sperm suspension preparation
23.45 + 2.7
12.18 + 2.14
males used in the first set of analyses (see sample sizes for
each species in the electronic supplementary material, table S1),
we incubated sperm suspensions at 378C in mT-H medium
under air for 3 h as described previously [34]. We then snapfroze each sperm sample in liquid nitrogen and stored the
samples at 2808C until SCSA analysis.
Males were sacrificed by cervical dislocation and weighed. Testes
were removed and weighed. Mature spermatozoa were collected
from the caudae epididymides and vasa deferentia. In several
species, DNA fragmentation has been shown not to differ between
the spermatozoa obtained from the cauda and ejaculated spermatozoa [32]. We placed the tissue in a Petri dish containing medium
prewarmed to 378C, made several cuts and allowed spermatozoa
to swim out for a period of 10 min. We used two types of media
depending on the experiment, one being a non-capacitating
medium (Hepes-buffered modified Tyrode’s medium, mT-H)
and the other a capacitating medium (mT-HB, containing 15 mM
NaHCO3, equilibrated with 5% CO2) [33]. mT-H supports sperm
survival but not the development of changes required for fertilization (i.e. ‘capacitation’, the physiological switching on of
spermatozoa required for sperm–oocyte interactions), whereas
mT-HB supports capacitation and the ensuing hyperactivation
[33]. After 10 min of incubation for swim-out, the sperm suspension (approx. 10– 20 106 spermatozoa ml21) was transferred
to a prewarmed Eppendorf tube. Each sperm suspension was
maintained at 378C until processing.
We used males (n ¼ 5 per species) from three species of Mus that
differ greatly in their levels of sperm competition: M. musculus,
M. spretus and M. spicilegus. These three species have been characterized as a good model for studies on sperm competition in
rodents, representing low, intermediate and high levels of sperm
competition, respectively [6]. For each male, we incubated spermatozoa from one cauda epididymis under non-capacitating
conditions (medium mT-H, under air, at 378C) and spermatozoa
from the other cauda under capacitating conditions (medium mTHB, under 5% CO2/air, at 378C). In both cases, spermatozoa were
incubated for 90 min. Sperm samples were taken at time 0 and at
90 min. For each male, we thus collected four different samples:
spermatozoa incubated under non-capacitating conditions at time
0 and after 90 min, and spermatozoa incubated under capacitating
conditions at time 0 and after 90 min. All samples were snap-frozen
in liquid nitrogen and then stored at 2808C until SCSA analysis.
(c) Comparative analyses
(e) Effects of physical and chemical stress
We performed two sets of comparative analyses. In the first set of
analyses, we quantified the baseline levels of sperm DNA fragmentation in 18 rodent species (see sample sizes in table 1). We
collected a subsample of sperm suspension from each male
immediately after sperm swim-out (10 min after reproductive
tract was excised). We snap-froze each sperm sample in liquid
nitrogen and stored frozen samples at 2808C until SCSA analysis. In the second set of analyses, we quantified the effect of
incubation for 3 h on sperm DNA integrity. For a subset of
We examined the effect of stressors on spermatozoa from the
three mouse species with different sperm competition levels, as
above, namely M. musculus, M. spretus and M. spicilegus (n ¼ 5
per species). For each male, we collected spermatozoa from
both caudae in mT-H medium under air, allowing sperm to
swim out for 10 min. We divided the resulting sperm suspension
in 12 aliquots. One aliquot was directly snap-frozen in liquid
nitrogen without any treatment (control). Another aliquot was
snap-frozen in liquid nitrogen, thawed at room temperature,
(d) Effect of capacitation
Proc. R. Soc. B 283: 20152708
testes mass (g)
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species
3
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Chromatin stability was assessed by using the SCSA [23].
The SCSA is based on the susceptibility of the sperm DNA to
acid-induced denaturation in situ and on the metachromatic
properties of the stain acridine orange, which fluoresces green
when associated with the intact double-stranded DNA helix,
but red when associated with single-strand denaturated
DNA and RNA [22,36]. SCSA has been previously used in
rodent spermatozoa to determine DNA fragmentation with
good results [21].
Samples were thawed on crushed ice, diluted with TNE
buffer (0.15 M NaCl, 0.01 M Tris – HCl, 1 mM EDTA; pH 7.4) at
a final sperm concentration of 2 106 cells ml21, and 200 ml of
sperm suspension were placed in a cytometry tube. Immediately,
400 ml of an acid-detergent solution (0.08 N HCl, 0.15 M NaCl,
0.1% Triton X-100; pH 1.4) were added to the tube. After exactly
30 s, 1.2 ml of acridine orange staining solution (0.037 M citric
acid, 0.126 M Na2HPO4, 0.0011 M disodium EDTA, 0.15 M
NaCl; pH 6.0, 48C) containing 6 mg ml21 electrophoretically purified acridine orange were added. Stained samples were analysed
3 min later by flow cytometry. Acridine orange was excited by
using an argon laser providing 488 nm light. A total of 5000
events were accumulated for each sample. We expressed the
extent of DNA denaturation in terms of DNA fragmentation
index (DFI), which is the ratio of red to total (red plus green) fluorescence intensity (i.e. the level of denatured DNA over the total
DNA) [22]. The DFI value was calculated for each sperm cell in a
sample, and the resulting DFI frequency profile was obtained.
Total DNA fragmentation index (tDFI) was defined as the percentage of spermatozoa with a DFI value over 25. High DNA
stainability (HDS), which offers a measure of the percentage
of immature sperm cells, was defined as the percentage of spermatozoa with green fluorescence higher than channel 600 (of
1024 channels).
3.0
log tDFI
2.5
2.0
1.5
1.0
–1.0
–0.5
0
log RTS
0.5
1.0
Figure 1. Relationship between relative testes size (RTS, sensu [43]) and
total sperm DNA fragmentation index (tDFI). This representation does not
include the phylogenetic corrections included in the statistical models.
(g) Statistical analyses
All statistical analyses were conducted using R v. 3.1.0 [37]. Normality was checked with the Shapiro – Wilk normality test. If
normality was not met, we used a logarithmic transformation.
Average values are reported as mean + s.e.m. Significance level
(a) was set at 0.05 for all the tests.
For the comparative analyses of DNA fragmentation, we performed regressions using phylogenetic generalized least-squares
(PGLS) analyses to control for phylogenetic association [38]. All
PGLS analyses were performed using the caper v. 0.5.2 package
for R [39]. The PGLS analysis estimates a phylogenetic scaling
parameter lambda (l ), which is then incorporated in the
models to control for phylogenetic effects. The phylogenetic
reconstruction used in the PGLS analyses is included in the electronic supplementary material, figure S1. We used tDFI or HDS
as the dependent variable (given that HDS is a measure of the
degree of sperm maturation, we only analysed HDS in the first
series of analyses, but not in the subsequent analyses in which
spermatozoa were subjected to various treatments). We used
body mass and testes mass as predictors in the PGLS analyses.
This provided a measure of the relationship between each dependent variable and relative testes mass (RTS). This latter variable
has been shown to reflect sperm competition levels in rodents
[40 – 42]. Given that body mass and testes mass are related to
each other (i.e. they are non-orthogonal), a sequential (type I)
sum of squares was used, adding the two predictors to the
models in the following order: body mass, testes mass. For the
graphical representation of RTS (figure 1), and for the calculation
of RTS values in table 1, and only in these cases, RTS was calculated using Kenagy and Trombulak’s rodent-specific regression
equation: RTS ¼ testes mass/0.031 body mass0.77 [43].
For analyses comparing three Mus species, we performed
two-way repeated measures ANOVAs. We used tDFI as the
dependent variable, and species and treatment as factors (we
also considered the interaction between species and treatment).
The repeated measure was given by male identity.
3. Results
(a) Comparative analyses
The 18 species of rodents that we studied offer a large range
of body masses (from 5.84 + 0.48 g in M. minutoides to
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(f ) DNA fragmentation assessment
4
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vortexed during 10 s and snap-frozen again (vortex treatment).
The remaining ten samples were exposed to different concentrations of DNase-I and H2O2, in line with the concentrations
used for laboratory mouse spermatozoa in a previous study
[35]. The use of these concentrations of DNase-I and H2O2 provided the opportunity to directly compare our results with
those of a previous comparative study on mammals [35].
For the DNase-I treatment, five sperm aliquots in mT-H were
centrifuged at 1677g (5000 r.p.m. in a MiniSpin Plus, Eppendorf
Ibérica, Madrid, Spain) for 5 min. The resulting sperm pellet was
resuspended in 100 ml of permeabilizing solution (0.1% sodium
citrate and 0.1% Triton X-100, both from Sigma, Madrid, Spain,
in mT-H) and incubated for 2 min at 378C. This permeabilizing
solution allows DNase-I to cross the sperm membrane. After centrifugation at 1677g for 5 min and removal of the supernatant, we
added 500 ml of the corresponding DNase-I concentration (0, 1,
10, 100 or 1000 U ml21 DNase-I from bovine pancreas expressed
in Pichia pastoris; Roche Diagnostics, Mannheim, Germany). After
each centrifugation, the sperm pellet was resuspended by lightly
flicking the end of the tube. After 30 min of incubation at 378C,
we centrifuged each sample twice at 1677g for 5 min, each time
removing the supernatant and adding new mT-H. Samples
were then snap-frozen in liquid nitrogen.
For the H2O2 treatment, we first centrifuged five sperm aliquots in mT-H at 1677g for 5 min, then removed the
supernatant and added 500 ml of the corresponding H2O2 solution (0, 0.1, 1, 10 or 100 mM H2O2 in mT-H) made from a
commercial solution (35%, w/w; Sigma). After 1 h of incubation
at 378C, we centrifuged each sample twice at 1677g for 5 min,
each time removing the supernatant and adding new mT-H.
Samples were then snap-frozen in liquid nitrogen until DNA
fragmentation analysis by means of SCSA.
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In the overall model, we found significant differences among
the four treatments (control 0 min, control 90 min, capacitation
0 min and capacitation 90 min; F3,36 ¼ 4.18, p ¼ 0.01; figure 2).
In subanalyses by species, there were no significant differences
between treatments in M. musculus (F3,12 ¼ 0.73, p ¼ 0.55),
whereas significant differences were detected in M. spretus
(F3,12 ¼ 5.7, p ¼ 0.01) and M. spicilegus (F3,12 ¼ 5.6, p ¼ 0.01).
Values of tDFI were higher after 90 min of incubation
under capacitating conditions in both M. spretus (capacitation
0 min versus capacitation 90 min: F1,4 ¼ 13.4, p ¼ 0.02) and
M. spicilegus (F1,4 ¼ 14.2, p ¼ 0.02; figure 2). However, we
found no difference between 90 min of incubation under
non-capacitating and capacitating conditions (control
90 min versus capacitation 90 min: M. spretus: F1,4 ¼ 3.57,
p ¼ 0.13; M. spicilegus: F1,4 ¼ 0.003, p ¼ 0.96).
(c) Effects of physical and chemical stressors
There was no increase in tDFI due to a vortex treatment (twoway repeated measures ANOVA: F1,12 ¼ 0.1, p ¼ 0.76; electronic
supplementary material, figure S2a). Also, there were no differences in tDFI between the three species with this treatment
(F2,12 ¼ 0.54, p ¼ 0.6).
There were no significant differences in tDFI values
between treatments with various DNase-I concentrations
(F4,48 ¼ 2.23, p ¼ 0.08) or among the three different species
(F2,12 ¼ 1.01, p ¼ 0.4; electronic supplementary material,
figure S2b).
4
tDFI
3
5
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(b) Effect of capacitation
condition
control 0
control 90
capacitation 0
capacitation 90
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111.02 + 8.59 g in Me. auratus), testes masses (from 0.08 +
0.01 g in M. castaneus to 3.32 + 0.23 g in Me. auratus) and relative testes sizes (RTS; from 0.26 + 0.02 in M. castaneus to
3.83 + 0.11 in C. griseus). Values for body mass, testes mass,
RTS, tDFI and HDS from these 18 species are summarized
in table 1.
We did not predict a priori any relationship between body
mass and either tDFI or HDS values. However, we found that
larger species had higher tDFI values (PGLS: F1,15 ¼ 23.2,
p ¼ 0.0002) and higher HDS values (PGLS: F1,15 ¼ 8.12,
p ¼ 0.01).
We obtained baseline levels for tDFI and HDS from the
18 species. We found that higher RTS values were associated
with higher tDFI values (PGLS: F1,15 ¼ 6.8, p ¼ 0.02; figure 1)
but that RTS did not relate to HDS (PGLS: F1,15 ¼ 20.15,
p ¼ 0.89; table 1).
To determine the effect of incubation in non-capacitating
medium (a condition that resembles that experienced by spermatozoa in the female reproductive tract), we assessed sperm
tDFI and HDS after 3 h of incubation in 17 species (we were
not able to obtain 3 h incubation samples from A. sylvaticus;
tDFI and HDS values at 0 h and at 3 h of incubation are
reported in the electronic supplementary material, table S1,
although only tDFI data were analysed at 3 h, as explained
above). Similar to the baseline results, after 3 h of incubation
there was also a positive relationship between tDFI and RTS
(PGLS: F1,14 ¼ 5.55, p ¼ 0.03). However, the increase in
tDFI after 3 h of incubation was similar across species and
thus was not related to RTS (PGLS: F1,14 ¼ 1.12, p ¼ 0.31;
electronic supplementary material, table S1). Thus, the
relationship between RTS and tDFI at 3 h was construed by
the baseline differences between species and there were no
differential effects of incubation time on changes in tDFI.
2
1
0
musculus
spretus
species
spicilegus
Figure 2. Effect of capacitation on sperm DNA fragmentation. Spermatozoa
from M. musculus, M. spretus and M. spicilegus were incubated in noncapacitating (control 0 and control 90) or capacitating (capacitation 0 and
capacitation 90) media during 10 þ 0 min (control 0 and capacitation 0)
or 10 þ 90 min (control 90 and capacitation 90). tDFI, total sperm DNA fragmentation index. Mean and s.e.m. are presented (n ¼ 5 for each species).
Similarly, there were not significant differences in tDFI
depending on H2O2 concentrations (F4,48 ¼ 0.67, p ¼ 0.62) or
among the three different species (F2,12 ¼ 3.8, p ¼ 0.06;
electronic supplementary material, figure S2c).
4. Discussion
We found a positive relationship between levels of sperm
competition and sperm DNA fragmentation across rodent
species. This is the first time that an increase in sperm competition levels has been found to be associated with a functional
cost involving sperm DNA integrity. This is a very important
cost if the spermatozoa that reach the fertilization site contain
damaged DNA, because spermatozoa exhibiting a significant
degree of DNA fragmentation are still capable of fertilization
[44]. It has to be considered that the increased numbers of
spermatozoa with damaged DNA in species with high
levels of sperm competition may not be part of the small
sperm subpopulation that reaches the site of fertilization if
any mechanism is in place in the female tract that aims
towards selection against damaged sperm. This hypothesis
could be tested in the future by quantitating sperm DNA
integrity in sperm subpopulations to assess if the increased
sperm DNA damage in species with high levels of sperm
competition is spread across the whole sperm population
or, instead, a particular subpopulation is affected. If the
latter, it may be that high levels of sperm competition lead
to an increased intra-male variation in sperm subpopulations,
with subpopulations in the first sections of the female reproductive tract having higher levels of sperm DNA damage
compared with sperm subpopulations reaching the last
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6
Proc. R. Soc. B 283: 20152708
DNA integrity is not affected by incubation in capacitating
medium [56], so all the cellular changes that occur during capacitation do not necessarily generate a negative impact on sperm
DNA across species. In addition, no differences were observed
between incubation in non-capacitating and capacitating
media. That is, the increase in DNA damage that we observed
in capacitating medium may not be related as much to the process of capacitation as to spermatozoa simply being incubated
(i.e. motile).
Physical stress (i.e. freezing–thawing followed by a vortex
treatment) did not result in any significant increase in sperm
DNA damage in mice. Similarly, mechanical stress in other
species as different as deer and dog does not lead to DNA
damage [57,58], which suggests that mammalian sperm DNA
can withstand some substantial amount of artificial physical
stress and must also be very robust to any naturally occurring
mechanical stress. By contrast, cryopreservation can lead to
sperm DNA damage in some species such as ram, boar and
alpaca [53,59,60], but not in others such as human, horse, bull
and dog [58,61–63].
We found that different concentrations of DNase-I and
H2O2 did not produce any significant sperm DNA fragmentation in three species of mice. These results are in line with
those previously reported in a study on spermatozoa of the
laboratory mouse [35]. In that study [35], spermatozoa from
mice, along with those of humans and bulls were exposed
to concentrations of DNase-I and H2O2 that were similar to
the ones that we used. Human and bull spermatozoa were
found to be very sensitive to both DNase-I and H2O2,
whereas mouse DNA spermatozoa were only moderately
affected by the highest concentrations of those stressors. We
found no significant effects on mouse sperm DNA integrity
even at the highest concentrations of DNase-I and H2O2. It
is worth noting that in the study by Villani et al. [35], spermatozoa were frozen and thawed and then exposed to each
treatment, whereas we treated fresh spermatozoa, which we
then froze until thawing for SCSA analysis. Such methodological differences could account for the small differences
between the two studies. In Villani et al.’s work [35], any
membrane damage or decrease in compaction due to freezing
and thawing may have rendered sperm DNA more accessible
to chemical stressors. If so, we can argue that our methodological approach closer resembles the conditions that
spermatozoa may encounter in the female reproductive
tract, and that spermatozoa in these mouse species are very
resistant to external chemical stressors.
In yet two other comparative studies—one examining spermatozoa of human, laboratory mouse and wallaby [29], the
other comparing spermatozoa of human, boar, laboratory
mouse, wombat, koala and grey kangaroo [64], and both
studies using alternative methods to SCSA to assess sperm
DNA fragmentation—the spermatozoa of mice were reported
to be highly resistant to oxidative stress. In both of these
studies, the sperm DNA of marsupial species were more sensitive to oxidative stress than the sperm DNA of eutherian
species. This higher sensitivity to oxidative stress is likely to
be due to a lack of disulfide cross-linking in marsupial sperm
chromatin. The oxidation of thiols to disulfides during chromatin condensation in eutherian mammals can provide not only
stability but also protection against genotoxic factors. Intracellular and seminal antioxidants may also protect sperm
DNA from oxidative stress. For example, cryopreserved spermatozoa from red deer that were treated with H2O2 suffered
rspb.royalsocietypublishing.org
sections of the female reproductive tract. Another untested
hypothesis is that the repair of DNA fragmentation that happens within the fertilized oocyte [45] may be more efficient or
increased in species with high levels of sperm competition,
leading to a coevolutionary process that may compensate
for high levels of sperm DNA damage in such species.
The higher prevalence of sperm DNA fragmentation in
species with high levels of sperm competition could be due
to higher levels of oxidative stress in these species. Recently,
it has been shown that the increase in sperm swimming velocity observed in rodent species that experience high levels of
sperm competition is driven to a great extent by a rise in the
content of sperm ATP [20,34,46]. That is, in species with high
levels of sperm competition there seem to be metabolic changes
and an increase in metabolism that allow sperm cells to swim
faster [47]. This increased metabolism may lead to a rise in
the production of ROS, which can damage not only lipids
and proteins but also the DNA in sperm cells. In species with
high levels of sperm competition there is a decrease in the
percentage of polyunsatured fatty acids in the sperm membrane, which confers lipid protection against ROS [48,49].
According to our results, analogous measures (e.g. changes
in protamination or compaction) appear not be in place in
these species to counteract the damaging effects of ROS on
sperm DNA. However, differences in the type and extent of
protamination in relation to levels of sperm competition
deserve future examination [50].
High DNA stainability (HDS) was higher in large-bodied
species but was not associated with different levels of sperm
competition. Absolute body size was also positively associated with higher levels of sperm DNA fragmentation (i.e.
higher tDFI values). The significant relationship between
body size and sperm DNA fragmentation across rodent
species could be related to species differences in mass-specific
metabolic rate [3], but further research is needed to clarify the
meaning of such association.
High levels of sperm competition lead to faster spermatogenesis and an increase in sperm production [3]. Such
enhanced sperm production does not seem to result in a
higher proportion of immature spermatozoa (as measured
by HDS) but could be partly responsible for the observed
increase in sperm DNA fragmentation. Indeed, our incubation experiment indicates that species differences with
regard to sperm DNA fragmentation may take place not
after but before ejaculation (i.e. during spermatogenesis,
sperm transport in the epididymis and storage in the cauda
epididymis). We observed that after 3 h of incubation, there
was a slight increase in sperm DNA damage relative to baseline levels, but such increase was similar across species and
thus was not influenced by differing levels of sperm competition. Likewise, in many species as diverse as human, deer,
boar and rhinoceros, a period of incubation that reflects the
time that spermatozoa will remain in the female reproductive
tract before capacitation does not result in any substantial
increase in sperm DNA damage [51–55].
The process of sperm capacitation, including acquisition of
fast motility (hyperactivation) by spermatozoa, also led to a
small increase in sperm DNA fragmentation. Interestingly,
such increase in sperm DNA fragmentation after incubation
in capacitating medium occurred in the two species
with higher levels of sperm competition (i.e. M. spretus and
M. spicilegus) but not in the species with low levels of sperm
competition (M. musculus). In bulls, as in M. musculus, sperm
Downloaded from http://rspb.royalsocietypublishing.org/ on February 24, 2017
the Spanish Research Council (CSIC). All procedures were carried out
following Spanish Animal Protection Regulation RD53/2013, which
conforms to European Union Regulation 2010/63.
Funding. This work was supported by a Ramón y Cajal fellowship
from the Spanish Ministry of Economy and Competitiveness (RYC2011-07943) and a Marie Curie Career Integration Grant (PCIG11GA-2012-321888) to J.d.-T., a fellowship from Campus Cientı́fico y
Tecnológico de la Energı́a y el Medioambiente—Universidad de Castilla-La Mancha (CYTEMA-UCLM) to O.G.-Á., a Juan de la Cierva
fellowship from the Spanish Ministry of Economy and Competitiveness to M.T (JCI-2011-10381) and grants from the Spanish Ministry of
Economy and Competitiveness (CGL2011-26341 to E.R.S.R.,
CGL2012-37423 to J.d.-T. and AGL2013-48421-R to A.J.S.).
Acknowledgements. We are grateful to Annie Orth and François
Bonhomme (Institut des Sciences de l’Evolution de Montpellier,
CNRS-Université Montpellier 2, France) for facilitating access to
animals, Juan José Luque-Larena and Leticia Arroyo for support
with fieldwork, and Juan Antonio Rielo for managing the animal
facilities and Esperanza Navarro for animal care at the Museo
Nacional de Ciencias Naturales (CSIC). We thank Marı́a VareaSánchez, Alberto Vicens, Pilar Villar and Ester Sansegundo for
their help collecting, processing, incubating and assessing spermatozoa in the laboratory. We also thank Diana Fisher and two
anonymous referees for constructive criticism.
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Ethics. The research protocol was approved by the Ethics Committee of
Data Accessibility. Datasets have been uploaded to Dryad and are
available for download: http://dx.doi.org/10.5061/dryad.cf2q1.
Authors’ contributions. J.d.-T. designed the study, coordinated the study,
collected and analysed data, and drafted the manuscript; O.G.-Á. collected SCSA data; A.J.S. coordinated the study and collected SCSA
data; M.T. collected reproductive data; J.J.G. coordinated the collection of SCSA data; E.R.S.R. conceived the study, participated in the
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