Sperm design and variation in the New World blackbirds (Icteridae)

Post-copulatory sexual selection (PCSS) is thought to be one of the evolutionary forces responsible for the rapid and divergent evolution of sperm design. However, whereas in some taxa particular sperm traits are positively associated with PCSS, in other taxa, these relationships are negative, and the causes of these different patterns across taxa are poorly understood. In a comparative study using New World blackbirds (Icteridae), we tested whether sperm design was influenced by the level of PCSS and found significant positive associations with the level of PCSS for all sperm components but head length. Additionally, whereas the absolute length of sperm components increased, their variation declined with the intensity of PCSS, indicating stabilizing selection around an optimal sperm design. Given the diversity of, and strong selection on, sperm design, it seems likely that sperm phenotype may influence sperm velocity within species. However, in contrast to other recent studies of passerine birds, but consistent with several other studies, we found no significant link between sperm design and velocity, using four different species that vary both in sperm design and PCSS. Potential reasons for this discrepancy between studies are discussed

Postcopulatory Sexual Selection Is Associated with Reduced Variation in Sperm Morphology

The evolutionary role of postcopulatory sexual selection in shaping male reproductive traits, including sperm morphology, is well documented in several taxa. However, previous studies have focused almost exclusively on the influence of sperm competition on variation among species. In this study we tested the hypothesis that intraspecific variation in sperm morphology is driven by the level of postcopulatory sexual selection in passerine birds.Using two proxy measures of sperm competition level, (i) relative testes size and (ii) extrapair paternity level, we found strong evidence that intermale variation in sperm morphology is negatively associated with the degree of postcopulatory sexual selection, independently of phylogeny.Our results show that the role of postcopulatory sexual selection in the evolution of sperm morphology extends to an intraspecific level, reducing the variation towards what might be a species-specific 'optimum' sperm phenotype. This finding suggests that while postcopulatory selection is generally directional (e.g., favouring longer sperm) across avian species, it also acts as a stabilising evolutionary force within species under intense selection, resulting in reduced variation in sperm morphology traits. We discuss some potential evolutionary mechanisms for this pattern

The intermediate sperm type and genitalia of Zorotypus shannoni Gurney: evidence supporting infraordinal lineages in Zoraptera (Insecta)

The sperm ultrastructure and the male and female genital apparatus of Zorotypus shannoni were examined and documented in detail, mainly using transmission electron microscopy micrographs. The findings suggest an evolutionary trend shared with Z. hubbardi and Z. impolitus. The three species are characterized by enlarged mitochondrial derivatives and related modifications. Giant sperm are probably a synapomorphy of Z. hubbardi and Z. impolitus, whereas an intermediate condition of this feature is found in Z. shannoni. The monophyletic origin of Z. caudelli, Z. magnicaudelli, Z. huxleyi and Z. weidneri is suggested by characteristically modified axonemes. The presence of extra-acrosomal material is also an unusual feature for Zoraptera, but this condition also occurs in the majority of polyneopteran groups. The long and convoluted female spermathecal duct with secretory and duct-forming cells is a constant feature in Zoraptera. The enlarged seminal receptacle suggests an evolutionary link between the male genital structures and the sperm size on one hand, and the size of the female spermatheca on the other. The small and otherwise uniform group Zoraptera exhibits a remarkable variation of sperm types and genital structures, suggesting the impact of different types of selection. It is likely that cryptic female choice plays a major role in shaping the genital apparatus

Divergent mating patterns and a unique mode of external sperm transfer in Zoraptera: an enigmatic group of pterygote insects

A remarkable external sperm transfer is described for the first time in a species of a group of winged insects (Pterygota), the enigmatic Zoraptera. Mating and sperm transfer of two species of the order were examined in detail, documented, and compared with each other and with patterns described for other species belonging to the order. The behavior differs strikingly in Zorotypus impolitus and Zorotypus magnicaudelli. A copula is performed by males and females of the latter, as it is also the case in other zorapteran species and generally in pterygote insects. In striking contrast to this, males of Z. impolitus do not copulate but deposit small (100 μm in diameter) spermatophores externally on the abdomen of the female. Each spermatophore contains only one giant spermatozoon (3 mm long and 3 μm wide), a unique feature in the entire Hexapoda. External sperm transfer in Pterygota is a highly unusual case of evolutionary reversal. The very small relict group Zoraptera displays a uniform general morphology but exhibits very different reproductive structures and patterns of mating behavior. This may be an extreme form of a more general situation in insects, with a specific form of selection resulting in an accelerated rate of evolution in the reproductive system. © 2013 Springer-Verlag Berlin Heidelberg