Interspecific diversity of testes mass and sperm morphology in the Philippine chrotomyine rodents: implications for differences in breeding systems across the species

Abstract

The high diversity of native Philippine murid rodents includes an old endemic group, the chrotomyines, which are the sister group of the Australasian hydromyines. Herein we detail their interspecific diversity of relative testes mass (RTM) and sperm morphology. We find that in chrotomyines, as in the Australasian hydromyines, testes mass relative to body mass differs by an order of magnitude across the species and ranges from a large RTM in Soricomys and Chrotomys species to a small RTM in Apomys. Sperm morphology is associated with these findings, with individuals in species of Soricomys and Chrotomys producing relatively larger spermatozoa with a prominent apical hook and long tail, whereas, by contrast, the Apomys species have a sperm head that either has a very short or no apical hook and a shorter tail. These findings indicate coevolution of RTM with sperm morphological traits across the species, with the marked interspecific differences in RTM suggesting differences in the intensity of intermale sperm competition and hence breeding system. Thus, we hypothesise that species of Soricomys and Chrotomys that produce more streamlined spermatozoa with longer tails have a polyandrous or promiscuous mating system, whereas the Apomys species, which produce smaller and less streamlined spermatozoa, may exhibit monogamy.

Sperm competition and the evolution of sperm form and function in mammals

“…There seems to be coevolution between sperm form and sperm size. Thus, an increase in sperm head complexity is accompanied by an increase in total sperm length, as originally pointed out by our group (Roldan, Gomendio, & Vitullo, ), and both are coevolving under the influence of sperm competition (Breed et al., ; McLennan et al., ; Pahl et al., ).…”

“…the so‐called hook) also shows that both its length and angle are influenced by sperm competition (Immler, Moore, Breed, & Birkhead, ; Varea‐Sánchez, Tourmente, Bastir, & Roldan, ). A series of recent comprehensive analyses of Australian rodents, a group in which several appendices have evolved in the sperm head, showed that several head parameters associate with levels of sperm competition (Breed et al., ; McLennan, Lüpold, Smissen, Rowe, & Breed, ; Pahl et al., ).…”

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“…Other species in the Pogonomys and Mallomys divisions in the genera Abeomelomys, Anisomys, Xenuromys, and Mammelomys, as well as that of Lorentzimys, have a sperm head in which the ventral processes are entirely absent but the apical process is generally longer than that of most other hydromyine rodents. The presence of a single apical process is similar to that of sperm morphology of various other genera of murid rodents that occur in Southeast Asia and the Philippines (Breed & Yong, 1986;Breed & Musser, 1991;Pahl et al, 2018;Breed et al, 2019) including species that are members of the Chrotomys division in the hydromyine tribe as well as members of the Maxomys, Dacnomys, and Bunomys divisions of the Rattini (Rowe et al, 2019).…”

Testicular Structure and Spermatogenesis in the Naked Mole-Rat Is Unique (Degenerate) and Atypical Compared to Other Mammals

“…These authors explained that in the absence of sperm competition, spermatozoa seem to degenerate and sperm quality decreases dramatically but it is still sufficient to fertilize. Several recent studies on other mammalian and bird species similarly related the presence of a high percentage of abnormal spermatozoa to potentially relaxed sperm competition and a monogamous mating system (Breed et al, 2018; Humann-Guilleminot et al, 2018; Peirce et al, 2018). Dorman et al (2013) indicated that there seems to be co-evolution of the gametes to aid sperm-zona interactions in the Bandicoot rat ( Bandicota indica ), where a low level of sperm competition is also apparent.…”