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

Abstract

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.

Keywords: Cordylochernes scorpioides; atp8; OXPHOS; RNA-Seq; differential expression; mitochondria; mother's curse; sperm competition.

Fig. 1.

B2 sperm competitive advantage in Cordylochernes scorpioides from central Panamá. Bars represent the least squares mean (LSM) proportion of offspring sired by A versus B2 haplogroup males in two-male sperm competition experiments with either A or B2 females. LSMs for A haplogroup males are indicated in blue and LSMs for B2 haplogroup males in red. Error bars represent standard error of the means.

Fig. 2.

Mitochondrial genes significantly overexpressed in the testicular tissues of B2 haplogroup males. The box and whisker plots compare the levels of gene expression for the mitochondrial genes that were significantly overexpressed by the C. scorpioides B2 haplogroup (red) relative to the A haplogroup (blue).

Fig. 3.

Mitochondrial genes significantly overexpressed by A haplogroup males. The box and whisker plots compare levels of gene expression for the mitochondrial genes that were significantly overexpressed by the C. scorpioides A haplogroup (blue) relative to the B2 haplogroup (red).

Fig. 4.

A heatmap showing mean-centered expression levels of the 15 mitochondrial genes. The A and B2 haplogroup samples cluster into two distinct clades as indicated in the dendrogram.