Rapid turnover of the W chromosome in geographical populations of wild silkmoths, Samia cynthia ssp

Our previous studies revealed a considerably high level of chromosomal polymorphism in wild silkmoths, Samia cynthia ssp. (Lepidoptera: Saturniidae). Geographical populations of this species complex differ in chromosome numbers and show derived sex chromosome systems including Z0/ZZ in S. cynthia ricini (2n = 27/28; Vietnam), neo-Wneo-Z/neo-Zneo-Z in S. cynthia walkeri (2n = 26/26; Sapporo, Hokkaido) and neo-WZ₁Z₂/Z₁Z₁Z₂Z₂ in S. cynthia subsp. indet. (2n = 25/26; Nagano, Honshu). In this study, we collected specimens of S. cynthia pryeri in Japanese islands Kyushu, Shikoku and Honshu, with an ancestral-like karyotype of 2n = 28 in both sexes and a WZ/ZZ sex chromosome system, except for one population, in which females have lost the W chromosome. However, the S. cynthia pryeri W chromosome showed a very unusual morphology: It was composed of a highly heterochromatic body, which remained condensed throughout the whole cell cycle and of a euchromatin-like “tail.” We examined molecular composition of the W and neo-W chromosomes in S. cynthia subspecies by comparative genomic hybridisation and fluorescence in situ hybridisation with W chromosome painting probes prepared from laser-microdissected W chromatin of S. cynthia pryeri. These methods revealed that the molecular composition of highly heterochromatic part of the S. cynthia pryeri W chromosome is very different and lacks homology in the genomes of other subspecies, whereas the euchromatin-like part of the W chromosome corresponds to a heterochromatic part of the neo-W chromosomes in S. cynthia walkeri and S. cynthia subsp. indet. Our findings suggest that the curious WZ system of S. cynthia pryeri may represent an ancestral state of the Samia species complex but do not exclude an alternative hypothesis of its derived origin.