Phenotypic plasticity, the ability of an organism to alter its trait in response to environmental variations, is well-documented across various taxa and is considered a crucial mechanism for survival in the face of environmental changes. This phenomenon has been a key topic in evolutionary biology, with numerous studies conducted on the subject. However, comparative phylogenetic analyses of phenotypic plasticity involving multiple species remain limited, and the macroevolutionary processes underlying this plasticity remain unclear for many taxonomic groups.
In this study, we focused on the genus Hynobius, which has diversified extensively in Japan and includes species adapted to two distinct environments in their larval stages: lentic (still water) and lotic (flowing water) habitats. Among lentic species, two are known to exhibit phenotypic plasticity, with larvae reared under high-density conditions developing enlarged gapes, an adaptation for cannibalism. Our study aimed to investigate how this plasticity has evolved across the phylogeny of Hynobius by conducting experiments on nine species representing major lineages within the genus.
For the experiment, we collected egg clutches from nine Hynobius species across various regions of Japan and reared the hatched larvae under two conditions: single rearing and high-density rearing (with 13 delayed-hatching siblings) from the 10th day after hatching. We measured body length and gape width weekly for four weeks.
The results showed that, in most lentic species, larvae raised under high-density conditions had significantly larger gape widths relative to body length (i.e., relative gape width) compared to those reared alone. However, in the lotic species Hynobius hirosei, no significant differences in relative gape width were observed at any time point. These findings suggest that the plasticity to develop enlarged gapes under high-density conditions has been retained in many Hynobius lineages. However, the absence of this plasticity in H. hirosei, which lays eggs in lotic environments, suggests that this trait may have been secondarily lost in lotic species or evolved in parallel among multiple lentic lineages.