Sex Determination
Cytological studies of a number of species ofStrongyloidessuggested that
sex may be determined chromosomally (Nigon and Roman, 1952). This
has been confirmed molecularly inS. ratti, where it has been shown that
there is an XX/XO, female/male mechanism of sex determination (Harvey
and Viney, 2001; Box 1 in Fig. 6.1). This mechanism of sex determination
is fairly common among nematodes and is used by the model free-
living nematodeCaenorhabditis elegans. Extensive genetic analysis in
C. eleganshas shown that sex is actually determined by the ratio of the
number of X chromosomes to the number of autosomes, rather than by
the number of X chromosomesper se(Hodgkin, 1988), but whether this
in the case inS. rattiis not known.
In theS. rattilife cycle, XX parasitic females produce genetically male
(XO) and female (XX) larvae. The cytological details of how this occurs
are not known, though it may be similar to a system in aphids where a
modified form of mitotic parthenogenesis occurs, in which the X chromo-
some undergoes a specialized behaviour (Blackman, 1980).S. rattimale
larvae are only able to develop into free-living adult males. However,
female larvae have a further developmental choice (below) and can
develop directly into iL3s or indirectly into free-living adult females.
When the free-living adults mate, all the progeny inherit the one paternal
X chromosome, thereby ensuring that the iL3 progeny of this mating have
a female, XX, genotype. For this to occur, it requires either: (i) that all the
male’s sperm carry an X chromosome; or (ii) that any zygotes which do
not have an XX genotype do not survive (Harvey and Viney, 2001). Which
of these possibilities is correct remains to be determined. However, it
would seem more efficient, and hence more likely, for spermatogenesis to
ensure that all sperm carry an X chromosome, and, indeed, this occurs in
aphids (Blackman, 1974).Female Development
Chromosomally female L1 progeny of parasitic females have a develop-
mental choice between direct development into iL3s and indirect
development into free-living adult females (Box 2 in Fig. 6.1). The
molecular and genetic basis of these different developmental fates is not
known. However, it is likely to be similar to that which controls a similar
developmental choice in theC. eleganslife cycle (Riddle and Albert,
1997; see below).Environmental Control of Nematode Life Cycles 113