Chironomids
generally have no cytologically recognized sex
chromosomes, the polytene chromosome
complement of males and females appearing identical. However in some cases
there are inversion or
translocation sequences which occur only in males, and serve to mark the Y
chromosome (Beermann
1955; Acton, 1957; Martin 1962; Newman 1977). The most
differentiated case
of sex chromosomes
has been reported in Telmatogeton hirtus, where there is an
XY1Y2
system, in which part of the Y chromosomes has become heterochromatic
(Newman 1977).
The Y chromosome can also be identified in other cases
by sex linkage of
enzyme loci (Martin & Lee
1988b), or even by experimental induction of chromosomal rearrangements of
the Y chromosome (Martin
1974).
These data clearly indicate that sex determination is a male dominant
system. There have been
reports of female dominant sex determination in the genus
Polypedilum (Martin 1966; Porter
& Martin 1977) and also in certain North American populations of
Chironomus tentans
(Thompson 1971: Thompson & Bowen 1972). While this seems
to be correct for
Polypedilum,
the situation in C. tentans appears to result from a
misinterpretation of the data (Martin
& Lee 1984).
It is
difficult to accurately map the chromosomal locations of
these MD genes, but it appears that there may be a relatively small number
of sites (Fig. 1). The most common sites differ in
different geographic
regions, e.g. on arm F in Europe but near the CD centromere in Australia.
Often the MD site changes at speciation, and there is a possibility that MD
site may be polymorphic in some species, although it may also be that these
different MD sites actually indicate the presence of cryptic species
(Martin & Lee 1988a).
Recently a
spontaneous mutation in a laboratory strain of
C. 'jacksoni' has lead to the creation of a new sex chromosome - but
not a change in MD site. The MD is near the centromere of the CD
chromosome, but a whole arm translocation has converted the arm combination
from BF, CD to BD and CF, CD still carries the MD locus, so the
translocation effectively creates an X1X2Y sex
chromosome system
(Fig. 2). In balanced gametes both the CF and BD
chromosomes segregate together and may be passed to either
sex.
It seems
likely that the sex determination pathway will be
related to that of Drosophila melanogaster subsequent to the action
of the sexlethal gene, which does not appear to be involved in sex
determination in other groups of Diptera (Martin & Lee 2000).
Data from other insects indicate that it will be centred on
the doublesex gene (Ohbayashi et al. 2001; Hediger
et al. 2004; Cho etal. 2007). Cho
etal (2007) proposed that, where female is the default sex,
the female-specific splicing of doublesex is the default
situation, with transformer suppressing splicers of
doublex. In males, transformer is non-functional,
so that the splicers mediate the slicing of the male-specific variant
of doublesex. This fits quite well with the model
of sex determination in Chironomus proposed by Martin &
Lee (2000) which proposed that male development could be mediated by
genes which down regulate the tra genes or alter the
transcription of the doublesex gene to switch the pathway from
the default female development to that of male development.
These could be genes that act at different points in the
sexual development pathway. The original scheme of
Martin & Lee (2000) needs to be modified by the inclusion of the
transcription suppression genes, which become one of the possible
sources of mutation (to a constitutive allele) and the recognition
that tra is supressing these genes rather than activating
dsx (Fig. 3).
Another possibility is that the MD gene is associated with a
transposable element (TE), which can insert into particular sites on
the chromosome where
recognition sequences for the TE occur (Martin & Lee
1988a). Such a gene could be a non-functional allele of
tra, or a suppressor of tra.
We are
interested in cloning and identifying the MD gene that occurs on arm
G (Fig. 1). In order to achieve this we are accumulating
cloned genes that
have been mapped to the distal end of the arm. Part of
the strategy is to
sequence this region of the chromosome, to determine the relative positions
of these genes and to locate the MD gene in relation to them.
Another approach is to obtain clones of known sex determination genes
from Chironomus, to see where these map relative to known MD
sites. The location of tra would be of particular
interest.