Sperm competitive advantage of a rare mitochondrial haplogroup linked to differential expression of mitochondrial oxidative phosphorylation genes.

Maternal inheritance of mitochondria creates a sex-specific selective sieve through which mitochondrial mutations harmful to males but not females accumulate and contribute to sexual differences in longevity and disease susceptibility. Because eggs and sperm are under disruptive selection, sperm are predicted to be particularly vulnerable to the genetic load generated by maternal inheritance, yet evidence for mitochondrial involvement in male fertility is limited and controversial. Here, we exploit the coexistence of two divergent mitochondrial haplogroups (A and B2) in a Neotropical arachnid to investigate the role of mitochondria in sperm competition. DNA profiling demonstrated that B2-carrying males sired more than three times as many offspring in sperm competition experiments than A males, and this B2 competitive advantage cannot be explained by female mitochondrial haplogroup or male nuclear genetic background. RNA-Seq of testicular tissues implicates differential expression of mitochondrial oxidative phosphorylation (OXPHOS) genes in the B2 competitive advantage, including a 22-fold upregulation of atp8 in B2 males. Previous comparative genomic analyses have revealed functionally significant amino acid substitutions in differentially expressed genes, indicating that the mitochondrial haplogroups differ not only in expression but also in DNA sequence and protein functioning. However, mitochondrial haplogroup had no effect on sperm number or sperm viability, and, when females were mated to a single male, neither male haplogroup, female haplogroup nor the interaction between male/female haplogroup significantly affected female reproductive success. Our findings therefore suggest that mitochondrial effects on male reproduction may often go undetected in noncompetitive contexts and may prove more important in nature than is currently appreciated.

Simulated climate warming and mitochondrial haplogroup modulate testicular small non-coding RNA expression in the neotropical pseudoscorpion, Cordylochernes scorpioides.

Recent theory suggests that tropical terrestrial arthropods are at significant risk from climate warming. Metabolic rate in such ectothermic species increases exponentially with environmental temperature, and a small temperature increase in a hot environment can therefore have a greater physiological impact than a large temperature increase in a cool environment. In two recent studies of the neotropical pseudoscorpion, Cordylochernes scorpioides, simulated climate warming significantly decreased survival, body size and level of sexual dimorphism. However, these effects were minor compared with catastrophic consequences for male fertility and female fecundity, identifying reproduction as the life stage most vulnerable to climate warming. Here, we examine the effects of chronic high-temperature exposure on epigenetic regulation in C. scorpioides in the context of naturally occurring variation in mitochondrial DNA. Epigenetic mechanisms, including DNA methylation, histone modifications and small non-coding RNA (sncRNA) expression, are particularly sensitive to environmental factors such as temperature, which can induce changes in epigenetic states and phenotypes that may be heritable across generations. Our results indicate that exposure of male pseudoscorpions to elevated temperature significantly altered the expression of >60 sncRNAs in testicular tissue, specifically microRNAs and piwi-interacting RNAs. Mitochondrial haplogroup was also a significant factor influencing both sncRNAs and mitochondrial gene expression. These findings demonstrate that chronic heat stress causes changes in epigenetic profiles that may account for reproductive dysfunction in C. scorpioides males. Moreover, through its effects on epigenetic regulation, mitochondrial DNA polymorphism may provide the potential for an adaptive evolutionary response to climate warming.

Sisters' curse: sexually antagonistic effects constrain the spread of a mitochondrial haplogroup superior in sperm competition.

Maternal inheritance of mitochondria creates a sex-specific selective sieve with implications for male longevity, disease susceptibility and infertility. Because males are an evolutionary dead end for mitochondria, mitochondrial mutations that are harmful or beneficial to males but not females cannot respond directly to selection. Although the importance of this male/female asymmetry in evolutionary response depends on the extent to which mitochondrial mutations exert antagonistic effects on male and female fitness, few studies have documented sex-specific selection acting on mitochondria. Here, we exploited the discovery of two highly divergent mitochondrial haplogroups (A and B2) in central Panamanian populations of the pseudoscorpion Cordylochernes scorpioides. Next-generation sequencing and phylogenetic analyses suggest that selection on the ND4 and ND4L mitochondrial genes may partially explain sexually antagonistic mitochondrial effects on reproduction. Males carrying the rare B2 mitochondrial haplogroup enjoy a marked advantage in sperm competition, but B2 females are significantly less sexually receptive at second mating than A females. This reduced propensity for polyandry is likely to significantly reduce female lifetime reproductive success, thereby limiting the spread of the male beneficial B2 haplogroup. Our findings suggest that maternal inheritance of mitochondria and sexually antagonistic selection can constrain male adaptation and sexual selection in nature.

Constant diurnal temperature regime alters the impact of simulated climate warming on a tropical pseudoscorpion.

Recent theory suggests that global warming may be catastrophic for tropical ectotherms. Although most studies addressing temperature effects in ectotherms utilize constant temperatures, Jensen's inequality and thermal stress considerations predict that this approach will underestimate warming effects on species experiencing daily temperature fluctuations in nature. Here, we tested this prediction in a neotropical pseudoscorpion. Nymphs were reared in control and high-temperature treatments under a constant daily temperature regime, and results compared to a companion fluctuating-temperature study. At constant temperature, pseudoscorpions outperformed their fluctuating-temperature counterparts. Individuals were larger, developed faster, and males produced more sperm, and females more embryos. The greatest impact of temperature regime involved short-term, adult exposure, with constant temperature mitigating high-temperature effects on reproductive traits. Our findings demonstrate the importance of realistic temperature regimes in climate warming studies, and suggest that exploitation of microhabitats that dampen temperature oscillations may be critical in avoiding extinction as tropical climates warm.