Fertilization Modes And The Evolution of Sperm Characteristics In Marine Fishes: Paired Comparisons of The Externally And Internally Fertilizing Species

Background: Fertilization modes may affect sperm characteristics, such as morphology, velocity, and motility. However, there is surprisingly little information on how fertilization mode affects sperm evolution because several factors (e.g. sperm competition and phylogeny) are intricately intertwined with this factor when phylogenetically distant species are compared. Here, we compared sperm characteristics between six externally and four internally fertilizing marine fishes from three different groups containing close relatives, taking into account the level of sperm competition. We also analysed the relationship between relative testis mass (as an index of sperm competition level) and sperm characteristics.Results: Sperm head morphology was significantly longer in species with internal fertilization than in those with external fertilization, suggesting that a longer head is advantageous for swimming in viscous ovarian fluid or the complex ovarian structure. In addition, sperm motility differed between external fertilizers and internal fertilizers; sperm of externally fertilizing species were only motile in seawater, and sperm of internally fertilizing species were only motile in an isotonic solution. These results suggest that sperm motility has adapted according to the fertilization mode. In contrast, total sperm length and sperm velocity did not correlate with fertilization mode, perhaps because of the different levels of sperm competition. Relative testis mass is positively correlated with sperm velocity and negatively correlated with the ratio of sperm head length/flagellum length. This finding suggests that species with high levels of sperm competition have sperm that are fast and have relatively long flagella compared to head length. These results contradict a previous assumption that the evolution of internal fertilization increases total sperm length. In addition, copulatory behaviour with internal insemination may involve a large intromittent organ, but this is not essential in fish, probably due to the avoidance of water resistance.Conclusions: We propose a new scenario of sperm evolution in which internal fertilization increases sperm head length but not total sperm length and changes sperm motility. In contrast, sperm competition affects sperm length and velocity. Our findings provide a new perspective on the evolutionary biology of sperm in fish.

Effect of population density on relationship between pre- and postcopulatory sexual traits

Sexual selection theory states that the premating (ornaments and armaments) sexual traits should trade off with the postmating (testes and ejaculates) sexual traits, assuming that growing and maintaining these traits is expensive and that total reproductive investments are limited. Male-male competition and sperm competition are predicted to affect how males allocate their finite resources to these traits. Here, we studied relative expenditure on pre- and postmating sexual traits among 82 species for three mammalian orders with varying population density using comparative phylogenetic analysis. The results showed that population density affected sexual size dimorphism (SSD) in both Artiodactyla and Carnivora, but not in Primates. However, relative testis mass and sperm size were not affected by population density. Moreover, we did not find associations between the SSD and testis mass or sperm size in three taxonomic groups. The interspecific relationships between pre- and postcopulatory sexual traits did not change with increased population density. Our findings suggest that population density did not affect variation in the relationship between pre- and postcopulatory sexual traits for these three mammalian orders.

Variation in somatic condition and testis mass in Feirana quadranus along an altitudinal gradient

The competition for fertilization among sperm from different males can drive variation in male reproductive investments. However, the mechanisms shaping reproductive allocation and the resulting variations in reproductive investment relative to environmental variables such as resource availability and male-male competition remain poorly known in frogs. Here, we investigated inter-population variation in male somatic condition and testis mass across four populations of the swelled vent frog Feirana quadranus along an altitudinal gradient. We found that relative testis mass did not increase with altitude, which was inconsistent with previous predictions that an increase in latitude and/or altitude should result in decreased sperm production in anurans due to shortened breeding seasons and the decline in resource availability. We also found no increase in somatic condition and male/female operational sex ratio with altitude. However, the somatic condition exhibited a positive correlation with testis mass, which indicated the condition-dependent testis size in F. quadranus. Moreover, an increase of testis mass with increasing male/female operational sex ratio suggest that male-male competition can result in an increased intensity of sperm competition, thereby increasing testis mass.

Pre- and Postcopulatory Traits ofSalvatorMale Lizards in Allopatry and Sympatry

The reproductive traits of males are under influence of sexual pressures before and after copulation. The strength of sexual selection varies across populations because they undergo varying competition for mating opportunities. Besides intraspecific pressures, individuals seem to be subjected to pressures driven by interspecific interactions in sympatry. Lizards may vary their reproductive strategies through varying sexual characters, body size, gonadal investment, and sperm traits. We evaluated the reproductive traits, involved in pre- and postcopulatory competition, in allopatric and sympatric populations ofSalvatorlizards. We observed a spatial gradient of male competition among populations, with the following order: allopatric zone ofS. rufescens; sympatric zone; and allopatric zone ofS. merianae. Accordingly, variation in secondary sexual character, the relative testis mass, and the length of sperm component was observed between allopatry and sympatry in each species, suggesting differences in the investment of reproductive traits. However, we found that these twoSalvatorspecies did not differ in secondary sexual characters in sympatry. Interestingly, the trade-off between testes and muscle varied differently from allopatry to sympatry between theseSalvatorspecies, suggesting that the influence of social context on reproductive traits investment would affect lizard species differently.

Altitudinal variation in male reproductive investment in a polyandrous frog species (Hyla gongshanensis jingdongensis)

Competition for fertilization in multi-male group spawning can drive variation in male reproductive investment (i.e., testis mass and sperm size). Inter-population comparisons of variation in energetic availability and allocation along geographical gradients allow insights into the mechanisms shaping the reproductive investments of animals. Although inter-population differences in female reproductive investment (i.e., clutch size and egg size) have been studied extensively across a wide range of taxa, little information on variation in reproductive investment in males is available. Here, we studied altitudinal variation in testis mass and sperm length among three populations in the Jingdong tree frog (Hyla gongshanensis jingdongensis), a polyandrous species, in Yunnan Province in China. The results showed that individuals exhibited smaller testes at higher altitudes while testes mass was positively correlated with body size, body condition and age. Longer sperm length was observed in the middle-altitude population. Moreover, we found that sperm number was positively correlated with testes mass and sperm length. Our correlational findings suggest that environmental constraints at high altitude select for less investment in testes and offspring number.

Reproduction of the greater bulldog bat Noctilio leporinus (Chiroptera: Noctilionidae) in a mangrove area in southern Brazil

The reproductive pattern of the greater bulldog bat (Noctilio leporinus) was studied in southern Brazil from January to December 1999. The morphological characteristics of their reproductive organs were recorded monthly, through histological analysis of testes and ovaries. Those data were correlated with forearm size and body mass, as well as with external reproductive characteristics. The diameter of the seminiferous tubule increased proportionately to testis mass and body weight, and larger males had greater testis mass. Sexually mature males were recorded throughout the year. Ovary histology revealed that females ovulate between July and December. Lactating females were recorded in nearly every month of the year. Noctilio leporinus has a bimodal polyestrous pattern, with an increase in the recruitment of juveniles between January and May.