Robertsonian chromosome polymorphism of Akodon molinae (Rodentia : Sigmodontinae): analysis of trivalents in meiotic prophase

Akodon molinae (with 2n = 42-43-44 and an FN = 44) shows a remarkable polym orphism of chromosome 1 in natural and laboratory populations. Specimens 2n = 42, named single homozygotes (SH), have a chromosome pair I formed by tw o large metacentric chromosomes. Specimens 2n = 3, heterozygotes (Ht), have one chromosome I and two medium-sized subtelocentric chromosomes, 1a and 1 b, which are homologous with the long and short arms of chromosome 1 respec tively. Specimens 2n = 44 are double homozygotes (DH), with just two pairs of medium-sized subtelocentric chromosomes, 1a and 1b. Analysis of meiotic metaphases I and II showed that anomalous segregation occurs more frequentl y in spermatocytes carrying the 1a and 1b chromosomes. This would disturb g ametogenesis and other reproductive and developmental processes, producing a marked decrease in viability of DH individuals. There is, as yet, no sati sfactory explanation for these phenomena. To investigate structural element s which might explain such segregational anomalies, we have studied bivalen t and trivalent synapsis in pachytcne spermatocytes from SH, Ht and DH spec imens. Of a total of 80 spermatocyte nuclei microspread, the following resu lts were obtained: of 16 microspreads from two SH individuals, 20 autosomic bivalents plus the XY bivalent were observed: of 48 microspreads from thre e Ht individuals, 19 autosomic bivalents, I trivalent and an XY bivalent we re seen; and of the 16 microspreads from two DH individuals, 21 autosomic b ivalents plus the XY bivalent were found. Trivalents analysed showed comple te pairing between the short arms of 1a and 1b and having an apparently nor mal synaptonemal complex (SC) with lengths of 1 and 2.8 mum. The trivalent SC showed three telomeric ends, corresponding to arms: q1 and q1a; p1 and g 1b; and p1a and p1b, with attachment plates to the nuclear envelope of norm al organisation. None of the trivalents showed asynapsis or desynapsis betw een p1a and p1b, nor an association with the XY bivalent. In 70 % of sperma tocytes studied, the XY bivalent showed complete pairing between X and Y, w ith SC formation along the whole length of the Y chromosome. The remaining 30 % showed partial pairing, with an SC length which varied from the common end. Based on these findings and those of previous studies, we discuss: 1. - that the obliged configuration of the trivalent. with SC formation betwee n the short al ms of 1a and 1b, helps to assure a quasi normal segregation between 1, 1a and 1b in anaphase I of Ht meiosis; and 2.- that co-existence in trivalents of chromosomes 1, 1a and 1b in Ht individuals, breaks down t he structural and functional integrity of the short arms of 1a and 1b, prod ucing an accumulative damage which would also explain the decreased viabili ty of individuals bearing these chromosomes.