The X1X2Y sex chromosome system in the fish Hoplias malabaricus. II. Meiotic analyses.

The neotropical fish Hoplias malabaricus shows diversified cytotypes and may represent a group of distinct species. One of these cytotypes is characterized by 2n = 40 and 2n = 39 chromosomes in females and males, respectively, with a multiple sex chromosome system of the X1X1X2X2/X1X2Y type. The Y, representing a large chromosome in male karyotype, is derived from a translocation event between two biarmed chromosomes: one of them similar to X1 chromosome (no. 6) and another one similar to X2 chromosome (probably no. 20). Meiotic data (standard and synaptonemal complexes analyses) show 18 bivalents and one characteristic trivalent in pachytene and metaphase I spermatocytes, as well as two kinds of metaphase II cells with 19 and 20 chromosomes. The trivalent is formed by the Y, X1 and X2 chromosomes and usually presents a complete pairing in pachytene. However, trivalents with partially or fully asynapsed segments are also observed. These segments are assumed to be non-homologous regions of the X1 and X2 chromosomes without correspondence with the Y chromosome, which can heterosynapse. This behaviour of the sex trivalent leading to a fully paired structure, taken together with the close frequencies of the two spermatocyte types at metaphase II, suggests a normal pattern for male H. malabaricus meiosis, representing a stabilized multiple sex chromosome system in this species.