Abstract
Being subject to intense post-copulatory selection, sperm size is a principal determining component of male fitness. Although previous studies have presented comparative sperm size data at higher taxonomic levels, information on the evolution of sperm size within species is generally lacking. Here, we studied two house mouse subspecies, Mus musculus musculus and Mus musculus domesticus, which undergo incipient speciation. We measured four sperm dimensions from cauda epididymis smears of 28 wild-caught mice of both subspecies. As inbred mouse strains are frequently used as proxies for exploring evolutionary processes, we further studied four wild-derived inbred strains from each subspecies. The subspecies differed significantly in terms of sperm head length and midpiece length, and these differences were consistent for wild mice and wild-derived strains pooled over genomes. When the inbred strains were analyzed individually, however, their strain-specific values were in some cases significantly shifted from subspecies-specific values derived from wild mice. We conclude that: (1) the size of sperm components differ in the two house mouse subspecies studied, and that (2) wild-derived strains reflect this natural polymorphism, serving as a potential tool to identify the genetic variation driving these evolutionary processes. Nevertheless, we suggest that more strains should be used in future experiments to account for natural variation and to avoid confounding results due to reduced variability and/or founder effect in the individual strains.
Highlights
In promiscuous species, sperm size, as determined by male-male sperm competition and cryptic female choice [1,2,3], is a principal component of male fitness
The wild Mus musculus domesticus (Mmd) and Mus musculus musculus (Mmm) groups differed in terms of midpiece length (F1,54548.54, P,0.001), sperm head length (F1,54538.15, P,0.001), and head width (F1,5456.52, P50.01); in all cases, values were higher in Mmm as compared to Mmd (Fig. 2, Table 2)
Given that Mmd and Mmm differed in two sperm length components, it is not surprising that these two groups differed in total sperm length (F1,5456.67, P50.01)
Summary
Sperm size, as determined by male-male sperm competition and cryptic female choice [1,2,3], is a principal component of male fitness. Mus musculus musculus (Mmm) and Mus musculus domesticus (Mmd), diverged about 350,000 years ago [14] and colonized Europe with the spread of agriculture [15]. In regions where their populations abut, these subspecies form a long, narrow secondary hybrid zone stretching from Norway to the Black Sea [16, 17]. For most of their divergence time, the two species’ ranges were separated, allowing them to accumulate numerous genetic incompatibilities [18]. Identification of spermatogenesis as a male fitness component with rapidly evolving incompatibilities led us to ask whether sperm size differentiation occurred simultaneous to mouse subspecific divergence
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