Incomplete Synapsis and Chiasma Localization: The Chicken or the Egg?

In the present study, and as a sincere tribute from the Cytogenetics teams from Madrid to Professor Máximo Drets on his 80th birthday, we have analyzed and compared 3 different grasshopper species with different synaptic patterns, a standard pattern, a second pattern with synapsis restricted to the proximal regions, and a third pattern with synapsis restricted to the distal regions. In the 3 species we have thoroughly analyzed the relationships among cohesin axis morphogenesis, formation of double strand breaks (DSBs) and recombination initiation. Our results demonstrate that in every case recombination initiation precedes synapsis, and that there is a direct relationship between the absence of meiotic recombination and the existence of particular unsynapsed chromosomal regions during prophase I. Based on our results we propose and discuss the mechanisms underlying the existence of incomplete synapsis and the localization of chiasma in wild species.