Contributions of synaptonemal complex studies to avian cytogenetics

PIGOZZI M.I.

San Petersburgo

Conferencia; 23rd International Colloquium on Animal Cytogenetics and Genomics; 2018

Saint Petersburg Association of Scientist and Scholars

More than 40 years ago, the synaptic nature of the ZW pair in birds was established looking at the synaptonemal complexes in chicken oocytes. The ZW pair forms asynaptonemal complex with lateral elements of unequal lengths.The small pseudoautosomal region in the chicken remains elusive to full sequence characterization, but Z-autosome translocations and recombination nodules analyses at pachytene showed that this region comprises the terminal segment of Zp and Wp. Unlike mammals, thepseudoautosomal regions of birds are extremely variable in size, spanning over 80% of the W chromosome in the primitive ratites. The chromatin of the sexbivalent does not show signs of heteropicnosis during pachytene and, therefore condensation is not required for the pairing of the mainly heterologous Z and W chromosomes of Neognathae. Evidence comprising expression analysis of Z-linked genes and mapping of histone modifications seems to confirm that the chromatin of the ZW pair is not subjectto meiosis-specific condensation and silencing in chicken oocytes. Nonetheless,this issue needs more comprehensive analyses using pure fractions of oocytes tomeasure gene expression of Z-linked genes vs. autosomal genes at specificmeiotic stages. Recombination nodules and MLH1 focus mapping in whole pachytene nuclei revealed less interspecific variations of global recombination rates inbirds. The mean recombination rate of the birds studied so far is in the rangeof 1.7-2.6 cM/Mb, while in eutherian mammals it is 0.5-1.1 cM/Mb. The differential organization of the chromatin loops along the synaptonemal complexes in birdsand mammals might provide a structural basis for these differences inrecombination levels. The changing morphology of the ZW pair in the chicken and other birds withhighly differentiates sex chromosomes might be an excellent model to study the dynamics of synaptonemal complex proteins and itsrelationships with chromatin. Also, the spatial arrangement of recombination nodules particles found in birds and their relationship with synaptonemal complex components could be analyzed in depthusing more recent approaches, such as super-resolution microscopy incombination with immunolocalization.