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
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
Developmental process that gives rise to males and females, which are distinct from the morphological, physiological and behavioural point of view
Regulation of expression of the genes located in the sexual chromosomes
to equalise the amount of their products to autosomal products in both sexes
Xm Xp Xp Am Ap zygote
Xp Xp
X 0; A A male development of the soma
Xp
X X; A A female development of the soma
Xp
Xm Xp Am Ap
germ cell testes
ovaries normal meiosis
X; Aoocytes meiosis I
Xp; Ap
Xm; Am
Am
X X; A sperm meiosis II
Life cycle of Sciara
Primitive mechanism
females males
XX X0
females males
females males
XX X0
X XX
XXX
X XX
XX X0
Sciara mechanism
?
non-disjunction of X-chromatids = gene c
maternally controlled X-elimination = gene e one male germ cell = one spermatocyte = gene s
X X 0
Xs
Xc
Xe Xe
Xs
Xc Xc
Xe
Xs
Xs
Xe
Xc Xe
Xc
Xs
Xc
Xs
Xe
Putative evolutionary pathways
Population scenario
- Infinitely large
- Discrete generations - Effects on fitness
- No migration
- Females accept only one mating
- Males can mate more than one female - viability = no effect
- mating capacity = no effect & random mating - reproduction
- meiotic drive
XX/X0
XX/XXe X0/Xe0
XsXs/XsXes Xs0/Xes0
XscXesc/XescXesc Xesc0
XX/XXs/XsXs X0/Xs0
extinction
XX/XXc X0/Xc0
(e)
(s)
(c) (c)
(s)
(c)
Monogenic species (s)
(e) (e)
Gene “s” is expressed after 1º meiotic division
Gene “s” is expressed after 2º meiotic division
(c)
(c)
Conclusions
- 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 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)
- 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 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
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
- The sex-determining system in sciara is compatible with maternal but not paternal imprinting
