Evolutionary pathways for sex-determining mechanisms based on X-chromosome elimination: the sciarid system

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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

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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

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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

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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

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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

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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)

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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

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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

Conclusion

Figure

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References

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