Relationship between incomplete synapsis and chiasma localization

Abstract

One of the subjects within the meiotic field that has been actively investigated in the recent years is the temporal and functional relationships between meiotic recombination, cohesin loading and synaptonemal complex (SC) assembly. Although the study of meiotic mutants has shed some light, many questions remain to be answered. Here, we have studied this topic in the orthopteran Paratettix meridionalis, a species with telocentric chromosomes, which shows two unusual cytological features: pairing and synapsis of homologues during prophase I are restricted to the non-centromeric distal regions and extremely distal chiasma localization in metaphase I bivalents. In order to determine whether there is a relationship between both phenomena, we have used: (1) a spreading technique for following the ultrastructure of SC assembly and (2) immunofluorescence for SMC3 and SMC1alpha cohesin subunits, which mark the development of the axial element (a SC component); the histone gamma-H2AX, which mostly labels the sites of double-strand breaks; and the recombinase RAD51. Spermatocytes showed conspicuous polarization of both the maturation of cohesin axes and the initiation of meiotic recombination events. Consequently, it is proposed that maturation of cohesin axes, which begins in very distal regions, could drive the latter loading of recombinases to such regions. This restricted distribution of recombination events along homologues would finally be responsible for the incomplete pairing and synapsis observed in all autosomes of the complement and hence for chiasma localization.

Cohesins and Cohesin-Regulators in Meiosis

“…A similar case has been observed in Paratettix meriodinalis, another grasshopper species with an incomplete synapsis pattern. The spermatocytes of this species show a polarization of cohesin axes maturation and the initiation of meiotic recombination, suggesting that the early development of cohesin axis would drive the loading of recombination machinery (Viera et al, 2009b). Obviously the study of AEs proteins would clarify the real relationship among SC formation, cohesin axis development and recombination in this special organism but these examples show the relevance of a cohesin axis formed by at least SMC proteins not only in the beginning of meiotic recombination but also in synapsis progression.…”

Spatiotemporal Asymmetry of the Meiotic Program Underlies the Predominantly Distal Distribution of Meiotic Crossovers in Barley

“…This highly localized formation of chiasmata is not unique and has been reported in other species. One such case is the orthopteran Paratettix meridionalis, which invariably exhibits distal chiasmata (Viera et al, 2009). However, in P. meridionalis and other instances, such as the grasshopper Stethophyma grossum, it appears that the whole meiotic program of chromosome pairing, synapsis, and recombination are restricted to regions where CO/chiasma formation occurs (Calvente et al, 2005).…”

Relationship between Chromosomal Races/Subraces in the Brachypterous Grasshopper Podisma sapporensis (Orthoptera: Acrididae) Inferred from Mitochondrial ND2 and COI Gene Sequences

“…But is asynapsis a cause of the proximal chiasma suppression, or a consequence? In species with strict chiasma localization, it is usually accompanied by asynapsis, though in these cases no structural hybridity obstructs synapsis (Fletcher 1977;Del Cerro et al 1997;Viera et al, 2009). We predict that, when proximal chiasma suppression in Rb trivalents is observed, it will be in heterozygotes of Rb polymorphisms, rather than in hybrids (natural or otherwise) between different chromosomal races.…”