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Micro- and macrospermatid production induced by meiotically unstable b-chromosomes in two species of grasshoppers (Acrididae)

Claudio J. Bidau

Laboratorio de Genética, Departamento de Ciencias Biológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Ciudad Universitaria, 1428 Buenos Aires, República Argentina

ABSTRACT

The effects of B-chromosomes on abnormal (non haploid) sperm formation was investigated in two species of grasshopper, Dichroplus pseudopunctulatus and Sinipta dalmani. A small telocentric B-chromosome was studied in several males of the former species. The B showed a very irregular meiotic behavior when single, which included lagging at the first and second meiotic divisions. As a result, large numbers of microspermatids carrying the laggard were observed in 1B males. A male carrying two B's did not show any significant increase in microspermatid production, correlated with the regular meiotic behavior of the B bivalent. No increase in macrospermatid production was observed in B-containing males and a mechanism of elimination obviously operates upon the B-chromosome in this species. In S. dalmani two small B-chromosomes were found one of which induced faulty cell division after telophase I probably due to its abnormal meiotic behavior. This resulted in a very large proportion of diploid spermatids (about 50%) with a consequent reduction in fertility.
Effects of B-chromosomes on chiasma frequency were also recorded: the B of D. pseudopunctulatus significantly increased mean cell chiasma frequency while, unexpectedly, the B chromosomes of S. dalmani seemed to decrease chiasma frequency significantly.
The consequences of increased abnormal sperm production and chiasma frequency modification through the presence of B-chromosomes are evaluated in terms of evolutionary as well as cytological results.

Keywords: Microspermatid; macrospermatid; meiotically; b-chromosomes; species; grasshoppers.

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