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Evolutionary pathways for sex-determining mechanisms
based on X-chromosome elimination: the sciarid system
Lucas Sánchez
Centro de Investigaciones Biológicas (C.S.I.C.), Madrid
Sex determination
Developmental process that gives rise to males and females,
which are distinct from the morphological, physiological
and behavioural point of view
If
sex determination is based on chromosome differences,
one sex being homomorphic and the other heteromorphic
for ther sexual chromosomes (females = 2X and males X)
then
Dosage compensation
Regulation of expression of the genes located in the sexual chromosomes
to equalise the amount of their products to autosomal products in both sexes
Life cycle of Sciara
development of the soma
X 0; A A
male
Xp; Ap
Xp Xp
testes
meiosis I
Xm; Am
Am
meiosis II
X X; A
sperm
Xp
zygote
Xm Xp Xp
Am Ap
Xm Xp
Am Ap
germ
cell
Xp
ovaries
development of the soma
normal meiosis
X; A
oocytes
X X; A A
female
females
males
XX
Putative evolutionary pathways
X0
Primitive mechanism
X
X
females
Xs
Xe
Xc
Xs
Xe
Xc
Xc
Xs
Xe
Xe
Xc
Xs
Xe
Xc
Xs
Xc
Xs
Xe
0
males
?
females
males
XX
X0
one male germ cell = one spermatocyte = gene s
non-disjunction of X-chromatids = gene c
Sciara mechanism
X
XX
XXX
X
XX
maternally controlled X-elimination = gene e
XX
females
X0
males
Population scenario
- Infinitely large
- Discrete generations
- Effects on fitness
- viability = no effect
- reproduction
- mating capacity = no effect & random mating
- meiotic drive
- No migration
- Females accept only one mating
- Males can mate more than one female
extinction
(c)
XX/XXs/XsXs
X0/Xs0
(s)
(s)
(c)
XX/XXc
X0/Xc0
XX/X0
(e)
(e)
XX/XXe
X0/Xe0
(e)
(c)
(s)
Gene “s” is expressed
(c)
after 1º meiotic division
XsXs/XsXes
Xs0/Xes0
Monogenic species
Gene “s” is expressed
after 2º meiotic division
(c)
XscXesc/XescXesc
Xesc0
Monogenic species (Sciara coprophila)
Conclusions
- The sciarid sex determination system could evolved in the absence of positive selection upon
the individual viability and mating capacity (absence of sexual selection).
- The sex ratio acts as a driving force for the evolution of the sciarid sex determination
system, yet it depends on the phenotypic effect of the new gene.
- The most plausible evolutionary pathway of the sciarid sex determination system is:
1) maternal factor causing elimination of X chromosome
2) one male germ cell = one sperm (heterochronic regulation)
3) non-disjunction of X chromatids in spermatogenesis
- The evolutionary pathway of the sciarid sex determination system shows constrictions; i.e. the
fixation of a new gene in the population depends on its genetic structure (structure of the gene
system)
Maternal versus paternal imprinting
- The elimination of X chromosomes depends on a maternal factor
- Difference between maternal and paternal imprinting
- maternal imprinting = the imprinted chromosome is not eliminated
- paternal imprinting = the imprinted chromosome is eliminated
Conclusion
- The sex-determining system in sciara is compatible with maternal but not paternal imprinting