Origin and Evolution of the Neo-Sex Chromosomes in Pamphagidae Grasshoppers through Chromosome Fusion and Following Heteromorphization

Abstract

In most phylogenetic lineages, the evolution of sex chromosomes is accompanied by their heteromorphization and degradation of one of them. The neo-sex chromosomes are useful model for studying early stages of these processes. Recently two lineages of the neo-sex chromosomes on different stages of heteromorphization was discovered in Pamphagidae family. The neo-sex chromosome heteromorphization was analyzed by generation of DNA probes derived from the neo-Xs and neo-Ys followed with chromosome painting in nineteen species of Pamphagidae family. The homologous regions of the neo-sex chromosomes were determined in closely related species with the painting procedure and image analysis with application of the Visualization of the Specific Signal in Silico software package. Results of these analyses and distribution of C-positive regions in the neo-sex chromosomes revealed details of the heteromorphization of the neo-sex chromosomes in species from both phylogenetic lineages of Pamphagidae grasshoppers. The hypothetical mechanism of the neo-Y degradation was suggested. It includes expansion of different repeats from the proximal neo-Y chromosome region by inversions, spreading them towards distal region. Amplification of these repeats leads to formation of C-positive regions and elimination of the C-negative regions located between them.

B Chromosomes in Grasshoppers: Different Origins and Pathways to the Modern Bs

“…Probably, the distribution of repeat clusters along the chromosome arm followed by the elimination of euchromatic regions can start from the proximal or distal part of the chromosome, leading to the formation of a visible C-positive block in a corresponded chromosome region. A strong support of this hypothesis was provided by finding of the neo-Y degradation in Pamphagidae grasshoppers leading morphotypes of the neo-Ys that are similar to morphotypes of some studied B s [ 27 , 28 , 29 ].…”

“…For instance, C-negative regions contain more mobile elements than C-positive ones [ 35 , 36 ]. This allowed generating a microdissected DNA probe from C-negative regions that paints only C-negative chromatin [ 31 ] even in different closely related species [ 29 , 34 , 69 ]. However, in B s C-negative regions can be enriched with repeats that differ from dispersed repeats of C-negative regions of A s .…”

“…However, the frequency of the fusion of the autosome with the X chromosome in Pamphagidae grasshoppers appeared to be unusually high. The neo-Y chromosome in these grasshoppers undergoes a process of intensive degradation [ 27 , 29 , 40 , 77 , 78 , 79 ]. Different stages of the neo-Y evolution can be observed in species of this family.…”

“…C-banding and fluorescence in situ hybridization (FISH) were performed as described earlier [ 28 , 29 ].…”

Genome and Karyotype Reorganization after Whole Genome Duplication in Free-Living Flatworms of the Genus Macrostomum

“…Then only TEs or TEs together with adjacent DNA sequences were amplified. As a result, numerous repeat clusters or regions enriched for different repeats were distributed along the chromosome, leading to different copies of the original chromosome [41,42]. The next step likely included the loss of various euchromatic regions located between the repeat clusters or regions enriched for repeats.…”