Sperm competition promotes diversity of sperm bundles in Ohomopterus ground beetles

Abstract

Diversification of sperm morphology has been investigated in the context of sperm competition, but the adaptive significance of sperm bundles is still unclear. In analyzing 10 taxa of the genus Carabus subgenus Ohomopterus and one related Carabus ground beetles, we found that dimorphic sperm bundles occurred in most species with varied degrees of bimodality, whereas sperm were generally monomorphic. Comparative analyses with phylogenetically independent contrasts revealed that the sizes of large and small sperm bundles evolved more rapidly than, and were not correlated with, the length of sperm, suggesting more intense selection on sperm bundle sizes and their independent responses to different evolutionary forces. The size of large sperm bundles was positively correlated with male genital morphology (pertinent to displacement of rival spermatophores) and postcopulatory guarding duration as well as male body length, suggesting that larger sperm bundles have been favored when the risk of spermatophore displacement is high. Larger sperm bundles may be advantageous because of their ability to migrate more rapidly into the spermatheca. In contrast, no clear association was detected between the small sperm bundle size and mating traits despite its rapid diversification. The present study provides the first record of heteromorphic sperm bundles, the diversity of which may be promoted by sperm competition.

Male reproductive system and spermatogenesis of Limodromus assimilis (Paykull 1790)

“…However, the spermatozoon heads of C . insulicola are only attached at hyaline caps, called spermatodesmata [11, 12]. …”

“…Within the Carabidae, spermatozoa morphology has already been analysed in the taxa Cicindelinae [9], Scaritinae [10], Carabinae [11, 12], Pterostichini [13–17] and Platynini [18]. Cicindelidae and Scaritinae species do not form sperm bundles (spermiozeugmata).…”

“…Between different species, the length varies from 0.11 to 25.3 mm [17]. Carabus insulicola Chaudoir 1869 (Carabinae), endemic to Japan, forms heteromorphic sperm conjugates in which the spermatostyles, where spermatozoa associate, are of varying lengths [11, 12]. Until now, only in one Platynini species, Jujiroa estriata Sasakawa, spermiozeugmata were analysed.…”

Structure of the male reproductive accessory glands ofPterostichus nigrita(Coleoptera: Carabidae), their role in spermatophore formation

“…Like other members of the tribe Pterostichini (Takami 2002(Takami , 2007Sasakawa 2006Sasakawa , 2007Takami and Sota 2007), male P. nigrita deposit a spermatophore, of unknown composition, containing sperm bundles (spermiozeugmata) that were formed in the vas deferens (Ferenz 1986;Hodgson et al 2012), in the bursa copulatrix of the female during copulation . The spermatophore may be formed by secretions from a pair of accessory glands that join the vasa deferentia which in turn open into the male's ejaculatory duct (Ferenz 1986;Hodgson et al 2012).…”

Sperm wars and the evolution of male fertility

“…In a variety of species, sperm can form conjugates of two or more individuals, who collectively swim faster than can Sperm wars individual sperm (Hayashi 1998, Moore et al 2002, Fisher & Hoekstra 2010. Among carabid ground beetles, the size of sperm bundles increases with traits known to be subject to selection from sperm competition, such as postcopulatory mate guarding and genital complexity (Takami & Sota 2007). In muroid rodents, the characteristic hooked morphology of the sperm head can be involved in the production of sperm conjugates, and the curvature of the hook increases with testes size, a widely used proxy for the strength of selection from sperm competition, suggesting that sperm conjugation may be an evolutionary response to selection via sperm competition in this group (Immler et al 2007a).…”