Chromosome core attachment to the meiotic nuclear envelope regulates synapsis in Chloealtis (Orthoptera)

Genetic evidence shows that in inbred families of Chloealtis conspersa, exceptional synapsis occurs between the distal ends of chromosomes 7 and 8 only if the other chromosome 7 carries a supernumerary (B) segment at its distal end. Cytological evidence shows that all the ends of chromosome cores are attached to the nuclear envelope and that synapsis is initiated close to the nuclear envelope. It is shown that distal synapsis of homologues 7 and 7B is perturbed by the B segment, which causes a misalignment of normally homologous regions at the nuclear envelope. It is proposed that this misalignment permits ends of heterologous chromosomes to undergo synapsis if they have mutual distal homologies (common in grasshoppers). In this manner the association between chromosome core ends and nuclear envelope regulates synapsis and chiasma formation in Chloealtis. Other organisms have different heterochromatic polymorphisms and pairing modes, but chiasma redistribution may result from a similar mechanism.