Evidence for True Fall-Mating in Japanese Newt Cynops pyrrhogaster

Institute for Amphibian Biology, Hiroshima University, Higashi-Hiroshima, Japan.
ZOOLOGICAL SCIENCE (Impact Factor: 0.86). 10/2011; 28(10):758-63. DOI: 10.2108/zsj.28.758
Source: PubMed


The mating season of Japanese newt Cynops pyrrhogaster is generally thought to occur once a year in spring to early summer, during the months of April to June, as in many other Japanese amphibians. However, in fall, from September to October, we often observed breeding colored males demonstrating a mating behavior with females in the field. In this study, in order to identify their true mating season, we anatomically and histologically investigated the annual maturation cycle of gonads and reproductive organs, including cloacal spermathecae in females, and, using a molecular marker, identified the seasonal origins of sperm, which are released in spring to perform insemination. We found that, in fall, ovaries are somewhat immature, while the testes were mature and the sperm already stored in the deferent ducts. Females stored a significant amount of sperm in around 80% of the spermatechae examined in October and 100% in December. When artificially ovulated in March before contact with male partners after hibernation, the females spawned fertilized eggs and these developed normally. Finally, we identified heterozygous genotypes of the visual pigment gene for the two different population types in the embryos, which were derived from a female who established contact with males of the same population in fall and then switched to males from another population until oviposition in spring. We therefore, conclude that the true mating season of this species occurs from fall to early summer, interrupted only by winter, and lasts six months longer (from October to June) than generally believed.

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    • "Several blocks to polyspermy operate to exclude the extra sperm outside the egg plasma membrane. In contrast, most urodeles exhibit internal fertilization and the female stores the sperm in a spermatheca near the cloaca (Akiyama et al. 2011 ). The eggs are inseminated by a small number of sperm released from the spermatheca just before oviposition. "
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    ABSTRACT: In amphibians, most urodeles (newts) exhibit polyspermy physiologically, but primitive urodeles (Hynobius) and anurans (frogs) exhibit monospermy. Several fertilizing sperm induce multiple small Ca2+ waves in the polyspermic egg, but a single large Ca2+ wave occurs in the monospermic egg. The Ca2+ waves in newt eggs are caused by a sperm-specific citrate synthase localized outside the mitochondria. The single Ca2+ wave at monospermy is necessary for eliciting a fast block to polyspermy, whereas the small multiple Ca2+ waves provide slower egg activation to permit the entry of several sperm at polyspermy. Physiological polyspermy seems to be evolved in association with the increase in size of eggs in urodeles, reptiles, and birds laying larger yolky eggs. The sperm factor (citrate synthase) operating in slower egg activation in polyspermic eggs is already prepared in the monospermic urodele Hynobius. We have focused on comparative studies in fertilization among amphibians to understand the role of egg activation in establishment of polyspermy with discussion of the evolution in vertebrates.
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