A phylogenetic study of sex determiner location in a group of Australasian Chironomus species (Diptera, Chironomidae)

Abstract

Sex determination in a group of phylogenetically related Chironomus species, of the pseudothummi complex, from south-eastern Australia and New Zealand is male heterogametic, controlled by a male determiner. The male determiner has been located at least to the level of the chromosome arm in most members of this phylogenetic group. It varies in location among many of the species and there are some phylogenetic patterns discernable, which are discussed in relation to the possible origin of the sex determiner. There is a group of species, Ch. oppositus ff. oppositus and whitei, Ch. australis, Ch. alternans a and Ch. alternans c, which appear to be central to this phylogeny, in which the sex determiner is located near the centromere of the CD chromosome, the most common location in the Australasian group. This is different from the most common location, arm F, of the thummi complex in Europe and North America. There is also a group, comprising Ch. oppositus f. tyleri, Ch. cloacalis, Ch. alternans b and Ch. nepeanensis, in which the sex determiner is on arm G. The arm A sex determiners, found in Ch. tepperi, Ch. oppositus ff. whitei and connori, and Ch. occidentalis, may be of common origin or they may be independently derived, as must be the arm B (Ch. duplex) and arm F (Ch. oppositus f. whitei) sex determiners. In Ch. oppositus f. whitei, four different chromosomal locations for the sex determiner have been identified. It is not yet clear whether these represent an unstable polymorphism or indicate the existence of cryptic subgroupings within this form. Although the location of the sex determiners can be assigned to particular chromosome arms, the precise location cannot be determined, therefore the assumption of common origin may not always be correct. Also, this uncertainty means that it is impossible at present to differentiate between a complex system of sex determination and the possibility of a translocatable sex determiner as explanations of the variability in sex determiner location. The forms of Ch. oppositus are redefined and renamed to avoid confusion caused by the previous names.