Abstract
B chromosomes (Bs) were described in most taxa of eukaryotes and in around 11.9% of studied Orthopteran species. In some grasshopper species, their evolution has led to many B chromosome morphotypes. We studied the Bs in nine species (Nocaracris tardus, Nocaracris cyanipes, Aeropus sibiricus, Chorthippus jacobsoni, Chorthippus apricarius, Bryodema gebleri, Asiotmethis heptapotamicus songoricus, Podisma sapporensis, and Eyprepocnemis plorans), analyzing their possible origin and further development. The studied Bs consisted of C-positive or C-positive and C-negative regions. Analyzing new data and considering current hypotheses, we suggest that Bs in grasshoppers could arise through different mechanisms and from different chromosomes of the main set. We gave our special attention to the Bs with C-negative regions and suggest a new hypothesis of B chromosome formation from large or medium autosomes. This hypothesis includes dissemination of repetitive sequences and development of intercalary heterochromatic blocks in euchromatic chromosome arm followed by deletion of euchromatic regions located between them. The hypothesis is based on the findings of the Eyprepocnemis plorans specimens with autosome containing numerous intercalary repeat clusters, analysis of C-positive Bs in Eyprepocnemis plorans and Podisma sapporensis containing intercalary and terminal C-negative regions, and development of heterochromatic neo-Y chromosome in some Pamphagidae grasshoppers.
Highlights
The B chromosomes (Bs ) were initially described in 1907 as additional elements to the standard karyotype in species of the Metapodius genus (Hemiptera) [1]
Two hypotheses were previously suggested for the mechanism of human small supernumerary marker chromosomes (SMCs) origin: SMCs arise as a result of deletion of almost the whole arm(s) or by forming the inverted duplication of pericentromeric region of acrocentrics followed by inactivation of one of the centromeres [33]
We suggest that different mechanisms can be involved in these processes
Summary
The B chromosomes (Bs ) were initially described in 1907 as additional elements to the standard karyotype in species of the Metapodius genus (Hemiptera) [1]. Bs were estimated from 1685 [2] in 1980 to more than 2828 [3] in 2017. Among Orthopteran species, Bs were described in 191 species, which constitutes 11,9% of studied species [4]. The presence of Bs is non-essential for organism development and provides little or no effect [5,6,7,8]. Despite intensive studies and wide range of used methods, the origin and further evolution of Bs remains an intriguing question. None of the approaches can provide compelling evidence on this topic
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