Fertilization modes and the evolution of sperm characteristics in marine fishes: Paired comparisons of externally and internally fertilizing species

Abstract

Fertilization mode may affect sperm characteristics, such as morphology, velocity, and motility. However, there is little information on how fertilization mode affects sperm evolution because several factors (e.g., sperm competition) are intricately intertwined when phylogenetically distant species are compared. Here, we investigated sperm characteristics by comparing seven externally and four internally fertilizing marine fishes from three different groups containing close relatives, considering sperm competition levels. The sperm head was significantly slenderer in internal fertilizers than in external fertilizers, suggesting that a slender head is advantageous for swimming in viscous ovarian fluid or in narrow spaces of the ovary. In addition, sperm motility differed between external and internal fertilizers; sperm of external fertilizers were only motile in seawater, whereas sperm of internal fertilizers were only motile in an isotonic solution. These results suggest that sperm motility was adapted according to fertilization mode. By contrast, total sperm length and sperm velocity were not associated with fertilization mode, perhaps because of the different levels of sperm competition. Relative testis mass (an index of sperm competition level) was positively correlated with sperm velocity and negatively correlated with the ratio of sperm head length to total sperm length. These findings suggest that species with higher levels of sperm competition have faster sperm with longer flagella relative to the head length. These results contradict the previous assumption that the evolution of internal fertilization increases the total sperm length. In addition, copulatory behavior with internal insemination may involve a large genital morphology, but this is not essential in fish, suggesting the existence of various sperm transfer methods. Although the power of our analyses is not strong because of the limited number of species, we propose a new scenario of sperm evolution in which internal fertilization would increase sperm head length, but not total sperm length, and change sperm motility.

Mating Season, Egg-Laying Season, and Internal Gametic Association in the Sympatrically Occurring Fluffy Sculpin (Oligocottus snyderi) and Rosy Sculpin (O. rubellio)

“…However, we also did not find any fertilized eggs and eggs of early cleavage stages in the ovaries of the Fluffy Sculpin, while the ovaries contained spermatozoa, indicating that copulation has taken place but fertilization has not occurred. Further, the sperm morphology of Fluffy sculpin is characterized as having an elongated head and motility in isotonic solution but immotile in seawater, similar to the sperm characteristics of the Rosy Sculpins and represented characteristics associated with IGA species (Abe and Muneahra, 2009;Koya et al, 2011;Ito and Awata, 2019;Ito et al, 2022). Therefore, although Morris (1956) has noted that the Fluffy Sculpins are internal fertilizers, we infer that their reproductive mode is IGA, as we observed in Rosy Sculpins.…”

“…PCR products were purified using Qiagen PCR purification kit and sequencing reactions were performed by ElimBio Pharmaceuticals (Hayward, CA). PCR amplification and sequencing of the Cytb locus were accomplished following Awata et al (2019Awata et al ( , 2022.…”

“…Gene tree topologies were constructed using MEGA version X (Stecher et al, 2020), with the maximum likelihood method with GTR+G+I model and 1,000 bootstrap replicates. For species identification, 11 CO1 sequences from ten species and 19 Cytb sequences from ten species were obtained from NCBI (Tables S1 and S2; Hastings and Burton, 2008;Buser and López 2015;Turanov and Kartavtsev, 2021;Awata et al, 2022). For both trees, Scorpaenichthys marmoratus was set as the outgroup.…”

“…Species with IGA share several characteristics with true internal fertilizers. For example, most IGA species have a well-developed genital papilla for mating (Evans and Meisner, 2009) and have sperm with elongated head, in comparison with external fertilizing species that lack copulatory anatomy and exhibit round sperm head morphology (Koya et al, 2011;Buser et al, 2017;Ito and Awata, 2019;Ito et al, 2022). While IGA is interesting in the evolution of reproductive strategy, it is currently unknown how widespread this strategy is within Agonidae and Psychrolutidae.…”

“…At present, 24 genera of Agonidae and Psychrolutidae are known to perform internal insemination spanning multiple independent clades (Agonidae: Agonomalus, Blepsias, Brachyopsis, Hemitripterus, Ocella, Pallasina, Podothecus;Psychrolutidae: Alcichthys, Artedius, Astrocottus, Bero, Chitonotus, Clinocottus, Enophrys, Furcina, Icelus, Oligocottus, Orthonopias, Pseudoblennius, Psychrolutes, Radulinopsis, Synchirus, Vellitor. See [Abe and Munehara, 2009;Munehara, 2011;Koya et al, 2015;Momota and Munehara, 2017;Awata et al, 2019Awata et al, , 2022). However, only seven species have been experimentally demonstrated to exhibit IGA by observation of initiation of development via exposing the eggs to seawater (Al.…”