ON THE MECHANISM OF HOMOLOGOUS SYNAPSIS IN LYCOSID SOLDERS

Lycosid spiders have 13 pairs of telocentric chromosomes and 2 nonhomo logous sex chromosomes in males. At leptotene, the kinetochore ends ar e attached to the nuclear envelope via thickened attachment plaques. H omologous synapsis begins at the attachment plaques and proceeds zippe r-like through the length of the synaptonemal complex, We have tested whether or not this simple form of homologous synapsis is obligatory b y inducing reciprocal translocations. Since we find many irradiated ce lls with quadrivalents at diakinesis-prometaphase and metaphase I, cle arly a backup system exists that can bring together homologous segment s disparate from each other in the nucleus. This mechanism apparently does not depend on end-initiated synapsis. Furthermore, we have found in previous studies that wolf spider bivalents are always unichiasmate , with either proximally or distally placed chiasmata. Since many chai n quadrivalents, but no ring quadrivalents, were seen in this study, c rossing over and chiasma placement obey a different set of rules when homologous segments are switched between chromosomes.