Abstract
Meiotic recombination occurs as a programmed event that initiates by the formation of DNA double-strand breaks (DSBs) that give rise to the formation of crossovers that are observed as chiasmata. Chiasmata are essential for the accurate chromosome segregation and the generation of new combinations of parental alleles. Some treatments that provoke exogenous DSBs also lead to alterations in the recombination pattern of some species in which full homologous synapsis is achieved at pachytene. We have carried out a similar approach in males of the grasshopper Stethophyma grossum, whose homologues show incomplete synapsis and proximal chiasma localization. After irradiating males with γ rays we have studied the distribution of both the histone variant γ-H2AX and the recombinase RAD51. These proteins are cytological markers of DSBs at early prophase I. We have inferred synaptonemal complex (SC) formation via identification of SMC3 and RAD 21 cohesin subunits. Whereas thick and thin SMC3 filaments would correspond to synapsed and unsynapsed regions, the presence of RAD21 is only restricted to synapsed regions. Results show that irradiated spermatocytes maintain restricted synapsis between homologues. However, the frequency and distribution of chiasmata in metaphase I bivalents is slightly changed and quadrivalents were also observed. These results could be related to the singular nuclear polarization displayed by the spermatocytes of this species.
Highlights
Meiosis determines the fate of chromosomes during the sexual life cycle by maintaining the chromosome number across generations in eukaryotes
During prophase I, spermatocytes of S. grossum show a remarkable nuclear polarization of the axis maturation of cohesin subunits, and the location of γ-H2AX, which marks the sites of double-strand breaks, and RAD51, which is involved in homology search
It was proposed that the restricted distribution of recombination events along the chromosomal axes is responsible for the incomplete presynaptic homologous alignment and, for the partial synaptonemal complex formation displayed by most bivalents[31, 38]
Summary
Meiosis determines the fate of chromosomes during the sexual life cycle by maintaining the chromosome number across generations in eukaryotes. It consists of two successive nuclear divisions preceded by one round of DNA replication and the formation of four haploid daughter cells. Recovery of Synapsis in the Grasshopper S.grossum after Exogenous DNA Double Strand Breaks chromosome segregation generate novel combinations of parental alleles and boost the genetic diversity of meiotic products. COs and sister chromatid cohesion are responsible for the correct bi-orientation of bivalents at metaphase I that leads to the segregation of a complete set of chromosomes at anaphase I. Sister chromatids separate at the second division generating four haploid meiotic products that originate the gametes. The fusion of gametes at fertilization restores the diploid chromosome number of the species [1]
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