Structure of eukaryotic chromosomes: the differences between mammalian (mouse), grasshopper (Stethophyma) and plant (Rosa) chromosomes as revealed at the diffuse stage of meiosis.

Abstract

The existence of a diffuse stage in both animal and plant meiosis has opened new possibilities to analyse the structure of the late pachytene and early diplotene chromosomes. When chromosomes from these stages are compared in phylogenetically remote organisms, basic differences in (1) the structure of the chromosomes and (2) the occurrence of interchromosomal connections are observed even with the light microscope only. In plants (Rosa), the homologous chromosomes have a thread-like appearance, and the early diplotene chromosomes first recontract very irregularly and are interconnected. The diplotene contraction then becomes more synchronized and the interconnections disappear. The meiotic chromosomes of the grasshopper (Stethophyma) have a multistranded appearance and are not interconnected, whereas the mitotic chromosomes seem to have centromere-to-centromere interconnections. In mammalian (mouse) meiosis, the pachytene chromosomes first acquire a lampbrush-like appearance, and the diplotene chromosomes first reappear as some kind of axes in the network of chromatin fibres, that gradually become organized around the axes. The meiotic and mitotic chromosomes are both interconnected. The plants, insects and mammals separated very early in their phylogenesis and in these phylogenetically separated branches the organization of the contracted chromosomes very probably developed independently and in a parallel manner.