Whereas nuclear genes are inherited from both parents, only females transmit mitochondrial DNA to offspring. Because males do not transmit mitochondria, heritability for mitochondrial-based differences in male survival and/or reproductive success is zero, precluding any direct evolutionary response to selection acting on mitochondrial mutations that either reduce or enhance male fitness. In this study, the arachnid, Cordylochernes scorpioides, will be used as a model system to investigate the effects of natural mitochondrial variation on the fertilization capacity of sperm. The research will encompass whole-mitochondrial genome sequencing, comprehensive analysis of physiological and morphological sperm characteristics important in fertilization success, an experiment designed to identify the target of selection acting on sperm traits, and a multi-generation study in which the evolutionary response to selection on sperm will be assessed using both maternally- and paternally-based selection regimes.
Uniparental inheritance of mitochondria creates male/female asymmetries in response to selection that have profound implications for evolutionary processes ranging from male adaptation to population viability, sexual selection and speciation, as well as for human health and disease. Mitochondrial mutations, including nucleotide substitutions, deletions and insertions, are known to be a primary cause of low sperm count and poor sperm motility in humans. The proposed research thus provides an exciting context for engaging both educators and students in basic evolutionary research with far-reaching practical implications.
In essentially all animal species, females differ fundamentally from males in transmitting not only genes in the cell nucleus but also genetic elements in the cell cytoplasm, including DNA in mitochondria and microorganisms such as bacteria and viruses. Because males do not transmit their mitochondria to offspring, mutations in the mitochondrial genome that are either beneficial or detrimental to males cannot respond directly to selection. Strict maternal inheritance limits the ability of selection to hone mitochondria for functioning in the physiological environment of the male, and may have wide-ranging implications for male fitness traits, including fertility, disease susceptibility and longevity. Using the Neotropical pseudoscorpion Cordylochernes scorpioides as a model system, we carried out a comprehensive investigation the evolutionary consequences of maternal inheritance of mitochondria for male fertility, vulnerability to mitochondrial-disrupting antibiotics, and environmental challenges posed by climate warming. As a live-bearing arthropod with indirect sperm transfer, C. scorpioides possesses a unique suite of reproductive and behavioral characteristics that makes it ideally suited for investigation of the effects of mitochondrial variation on male adaptation (Figure 1). This NSF-supported project produced three main outcomes. First, next generation DNA sequencing of whole mitochondrial genomes revealed the coexistence of two highly divergent mitochondrial lineages (A and B2) in central Panamanian populations of C. scorpioides (Figure 2). Our findings indicate that males carrying the rare B2 mitochondria enjoy a marked advantage in fertility but this male benefit is countered by maladaptive mating behavior on the part of B2-carrying females. Adaptive evolution analyses indicate that selection acting on the ND4 and ND4L mitochondrial genes may account for these sexually antagonistic mitochondrial effects (Figure 3), and significantly elevated levels of expression of these two genes in B2-male testicular tissue further implicate their involvement in mitochondrial-dependent differences in fertility. RNA sequencing studies revealed that small non-coding RNAs are abundantly expressed in C. scorpioides testicular tissue, with several of these regulatory RNAs exhibiting significant mitochondrial-associated differences in level of expression. This is one of the first studies to demonstrate opposing effects of naturally occurring mitochondrial genetic variation on reproductive success in males and females in an animal species. In a second study, we performed the first multigenerational study of the effects of chronic exposure to the antibiotic tetracycline on male and female reproduction function. Tetracycline is an inhibitor bacterial protein synthesis, and is also known to disrupt the translation of mitochondrial proteins. In experiment in which sibling C. scorpioides were randomly assigned to control and tetracycline treatments across replicate full-sibling families, tetracycline did not affect body size in either sex, female reproduction or sperm number. However, tetracycline-treated males exhibited significantly reduced sperm viability compared to control males, and transmitted this toxic effect of tetracycline on sperm to their untreated sons but not to their grandsons (Figure 4). These results are consistent with tetracycline-induced epigenetic changes in the male germline, and provide support for the hypothesis that the pernicious effects of antibiotics on spermatogenesis may at least partially explain the marked decline in human ejaculate quality over the last 60 years. In a third study, we assessed the life history, morphological and reproductive consequences of the 3.5 0C rise in temperature predicted for the tropics by the end of the century. Our results indicate that the phenotypic effects of elevated temperature are complex, differing markedly between the sexes and life history stages. Although survival, sex ratio, morphological traits and level of sexual dimorphism were significantly impacted by high temperature, these effects were relatively minor compared to the catastrophic consequences for reproduction (Figure 5). Exposure to high temperature during development dramatically reduced male fertility, and rendered females sterile. Our findings thus suggest that reproduction may be the Achilles’ heel of tropical arthropod species, as climate warming subjects them to an increasingly adverse thermal environment. In terms of broader impacts, the grant has thus far yielded eleven refereed scientific publications, six conference presentations, three master’s theses and one PhD dissertation, with additional manuscripts in preparation. Two publications have almetric scores in the 95th percentile of all journal articles surveyed, and several articles have been featured in scientific news outlets and the popular press. The grant supported two postdoctoral scholars, four graduate students, four Research Experiences for Undergraduates projects, and a total of 18 undergraduate students, including 14 women, five Asian Americans, three Hispanics, two Pacific Islanders, and one African American. Project participants achieved their career goals in fields ranging from basic research to K-12 instruction, medicine, and veterinary medicine, and have been placed in prestigious institutions, including Harvard Medical School, Stanford School of Medicine and the University of North Carolina. The project also involved K-12 outreach encompassing curriculum development and training of high school biology teachers and students in molecular genetic techniques and research design.
