Evolutionary reorganization of chromosome interposition in the space of trophocyte nuclei in closely related Drosophila species

One of the problems of systematics and phylogeny is the difficulty in establishing species affiliation in difficult-to-define groups of species (cryptic species), which are phenotypically similar to each other, but at the same time reproductively isolated and differ in genome structure. For most two-winged insects, the formation of giant polytene chromosomes with species-specific disc and interdisc structure is characteristic of the nuclei of salivary glands, trophocytes, and malpighian vessels. According to the structure of chromosomes species differ and phylogenetic trees between closely related species are built according to this feature. At the same time, there are species in which the structure of polytene chromosomes practically does not differ, such species are called homosequential. Therefore, it is difficult to establish phylogenetic relationships between these species. However, a new species trait has been discovered in malarial mosquitoes, by which homosequential species are also distinguished. This is the mutual arrangement of chromosomes in the space of trophocyte nuclei. In this connection, it is of great interest to study this phenomenon in other two-winged insects, especially in homosequent and cryptic species. In this connection, it was of great interest to study this phenomenon in other two-winged insects, especially in homosequent and in cryptic species. For the first time, the orientation of polytene chromosomes in the nuclei (nucleus morphotype) of the germinative cell system was analyzed in closely related species of the melanogaster subgroup and virilis group of the genus Drosophila. We have shown that a new species-specific trait, the nucleus morphotype, as well as that previously identified in malaria mosquito species, also occurs in closely related species of Drosophila. Four species-specific morphologic types of the nucleus were found in Drosophila: 1. With a local chromocenter; 2. With a diffuse chromocenter; 3. With chromosomes scattered in the space of the nucleus; 4. With chromosomes attached to the nuclear envelope (See Fig. 1). The regularity of distribution of species of melanogaster subgroup and virilis group by the feature “morphotype of trophocyte nuclei” correlating with phylogenetic centers of origin of species and geographical distribution of Drosophila species ranges above sea level was revealed. It was found that such factors as low temperature and inbreeding affect the synaptation of homologous chromosomes in the nuclei of ovarian trophocytes in D. melanogaster lines. It is possible that these factors may influence changes in the nucleus morphotype (spatial organization of chromosomes in the nucleus), which may further lead to the formation of a new species. Evolutionary transformations of the morphotype of trophocyte nuclei in species followed the scheme: from a local chromocenter in phylogenetically original species to its partial and then to its complete disappearance in derived species, while in a number of species chromosomes are dispersed in the nucleus and are in contact with the nuclear envelope. The article contains 1 Figure, 33 References. The Authors declare no conflict of interest.

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