What the distribution of sperm size can tell about the stability of spermatogenesis in hybrid frogs Pelophylax esculentus

Abstract

Interspecies hybrid frogs Pelophylax esculentus and one of its parental species Pelophylax ridibundus inhabit the Siversky Donets center of diversity of water frogs in Eastern Ukraine. These frogs can crossbreed and form progeny in population systems which are called hemiclonal (HPS). Such systems have their own exceptional features which make them interesting for studying. The Lower Dobrytskiy Pond, which is situated in the National Nature Park “Homilshansky lisy” and is a part of Siversky Donets river basin, is on focus. Current work is devoted to the combination of two methods of spermatogenesis investigation. First, using the method of Ag-staining we observed high variability of meiotic chromosomal plates in testes of 24 adult male water frogs P. esculentus (2n=26). Only one male had 100% of full meiotic plates with no aneuploid plates. A significant amount of studied males (21/24) produced aneuploid chromosomal plates (4-68% of the total amount of meiotic plates). This may lead to a decrease in their fertility or even to their entire sterility. Also, we have not observed any chromosomal meiotic plates in two of 24 males. Some males (8/24) even produced meiotic chromosomal plates with 26 bivalents (i.e. 4n germ cells) which may testify about the ability to produce diploid sperm. Further, the lengths of urinary sperm cells’ heads were measured. Finally, we performed an analysis of both meiotic chromosomes in testes and the distribution of sizes of urinary sperm cells’ heads of hybrid water frogs Pelophylax esculentus from Siversky Donets basin to find out if there is a link between these two features. No difference in sperm heads lengths was found between males producing moderate and low amounts of sperm. Based on the data of meiotic plates all males were assigned into five categories via PCA (principal component analysis). A significant difference in sperm heads lengths was found within the category I (males with mostly full meiotic plates). The analysed data shows that each male from the studied population has his own unique features. No direct link between sperm cells size and features of meiotic chromosomes in testes was found.