Determining the size of the male-specific region in the genome of the scuttle fly, Megaselia scalaris, a potential model system for the earliest stages of sex chromosome evolution

Abstract

Modern day whole-genome sequencing and the ability to make comparisons across many taxa have significantly advanced the study of sex chromosome evolution. The scuttle fly, Megaselia scalaris, is an appropriate model system for studying sex chromosome evolution exhibits sex chromosomes that are homomorphic in size, containing differences that are difficult to detect even microscopically. Presumably, sex in M. scalaris is determined by the presence or absence of a male-determining region, or sex realizer (M), that transposes among chromosomes at a low rate, essentially creating novel Y-chromosomes out of autosomes. M. scalaris would thus serve as a good model for exploring the primary stages of sex chromosome evolution. However, to serve as such a model, the identity and size of the male-specific region must be identified as a boundary within which to search for the sex realizer. This study focused on elucidating the relative size of the male-specific region in M. scalaris in relation to the genome. I compared previously generated genome sequences from male and female M. scalaris to isolate suggested regions unique to the male sex. Through a systematic approach involving sequencing, primer design, and PCR, I found the M. scalaris genome to be 27% male-specific, a preliminary result suggesting a large size inconsistent with published reports. This size further questions observations pointing to a relatively small male-determining region that transposes, but the male-specific region may be composed of additional elements unique to the male sex that do not ultimately determine sex. This study has refined the boundaries within which the sex realizer (M) of M. scalaris may be located. Future studies are necessary to pinpoint the sex realizer, an important step towards M. scalaris’ use as a model system in sex chromosome evolution.