Evolutionary process of a tetranucleotide microsatel- lite locus in Acipenseriforms

is ortodox in females but highly specialised in males, showing elimination of the paternally derived genome (1st division), and the non-disjunction of the two X chromatids and elimination of one chromatid of each autosome during the 2nd division. When the zygotic nuclei reach the egg cortex, one paternal X chromosome is eliminated in the somatic cells of embryos destined to be females (2X;2A) and two are eliminated in those destined to become males (X;2A). In the formation of the X/A chromosomal signal in sciarids an imprinting process occurs in one of the parents, which determines that the chromosomes to be eliminated are of paternal origin [2]. A maternal factor controls the number of X chromosomes eliminated by the zygote [3, 1, 4]. Therefore, the formation of the primary, chromosomal signal (2X;2A versus X0;2A) determining gender in sciarids is the consequence of four processes: lethality of non-X bearing sperm, non-disjunction of maternal-derived X chromatids during spermatogenesis, elimination (controlled by a maternal factor) of X paternal-derived X chromosomes in the embryo and chromosome imprinting. This work focuses on the putative evolutionary pathways that gave rise to sciarid sex determination system from the more ancient XX/X0 system, where the primary, chromosomal signal (2X;2A versus X0;2A) is a direct consequence of the chromosomal constitution of the gametes: oocytes are X;A and sperm are 0;A.