MALE MEIOSIS IN A RODENT WITH 2n=102 CHROMOSOMES, Tympanoctomys barrerae (OCTODONTIDAE).

LANZONE C.; BIDAU C.J; GALLARDO, M.H.; SANTOS J.L.

Wurzburg Alemanha

Conferencia; 14th International Chromosome Conference; 2001

The red viscacha rat, Tympanoctomys barrerae, is a desert specialist with a restricted distribution range near the Argentinian-Chile boundary. This species has a chromosome number of 2n=102 (100 + XY; 100 + XX), nuclear content is 16.8pg and the size of the sperm head is about 14um; gametic genome estimates indicate about 9.2 p. These figures have been interpreted as an indication of a tetraploid origin. We have analyzed the first meiotic division (zygotene, pachytene and diakinesis/metaphase I) in a male of T. barrerae through electron-microscopy of surface-spread synaptonemal complex (SC) and standard cytogenetic techniques. The pattern of pairing initiation and progression is described. No evidence of multivalent chromosome association was observed: male meiosis is regular with strict bivalent formation thus, if T. barrerae is polyploid, structural diploidization would have occurred during the evolution of its lineage. However, an intriguing feature is the existence of gross inequalities in length between the lateral elements of SCs of several autosomal bivalents which are especially evident at zygotene and early pachytene. These differences are adjusted as prophase I proceeds, but at diakinesis, between 8 an 11 autosomal heteromorphic bivalents were observed; otherwise, late prophase I is normal with chiasma frequencies ranging between 50 and 57. A single XY bivalent is observed in all meiotic cells. As in most mammals, T. barrerae shows partial synapsis of the X and Y chromosomes and in early pachytene the pairing and asynaptic regions are easily distinguishable. As pachytene progresses, both elements undergo splitting of the meiotic axes and each of them may show six or more axes of lesser width than that of the autosomes. In some pachytene nuclei, however, complete pairing of the axis, which in this case maintains the morphology of the autosomal axes, was observed, although in the synapsed region the X axis appears slightly thickened. At diakinesis the XY bivalent invariably displayed an end-to-end association of both sex chromosomes. The behaviour of the sex-chromosome axes of T. barrerae equals that of the Ctenomys, a phylogenetically related family, and could thus represent an ancestral condition.