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

Abstract

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.

The fate of W chromosomes in hybrids between wild silkmoths, Samia cynthia ssp.: no role in sex determination and reproduction

“…Two subspecies were collected at respective sites in 2005-2011 and reared on Alianthus altissima at the Field Science Center for Northern Biosphere (Hokkaido University, Sapporo, Japan; Yoshido et al, 2005Yoshido et al, , 2013.…”

“…Spread preparations of pachytene complements were made from gonads of both sexes as given in Yoshido et al (2013). Tissues of the last larvae were dissected in a saline solution, swollen for 10-15 min in a hypotonic solution (75 mM KCl) and then fixed for 10-15 min in Carnoy fixative (ethanol, chloroform, acetic acid, 6:3:1).…”

“…Fosmid probes were labelled using a Nick Translation Kit (Abbott Molecular Inc., Des Plaines, IL, USA) with Green-dUTP (Abbott Molecular Inc.) and Cy3-dUTP (GE Healthcare, Buckinghamshire, UK), respectively. For the detection of W chromosomes, we used FISH with W chromosome painting probes (the so-called type-2 W-probes), which highlighted the whole W chromosome in S. c. pryeri and a part of the neo-W chromosome in S. c. walkeri (Yoshido et al, 2013). The probes were prepared and then labelled with Cy3-dUTP (GE Healthcare) as described in Yoshido et al (2013).…”

“…The W chromosome in S. c. pryeri consists of two parts, the highly heterochromatic part (black in Figure 1) and less heterochromatic part (striped grey in Figure 1). The former part was found only in S. c. pryeri while the latter part corresponds to the ancestral segment (striped grey in Figure 1) of the neo-W chromosome in S. c. walkeri (Yoshido et al, 2013). Hence S. cynthia ssp.…”

“…indet. (Nagano, Japan) with 2n = 25♀/26♂, and neo-Wneo-Z/neo-Zneo-Z in S. c. walkeri (Sapporo, Japan) with 2n = 26♀/26♂ (Yoshido et al, 2005(Yoshido et al, , 2011a(Yoshido et al, , 2013. The variations in the constitution of sex chromosomes were derived from the loss of W chromosome and fusion between the sex chromosomes and autosomes, resulting in different chromosome numbers (Yoshido et al, 2011a, b;Sahara et al, 2012).…”