The B-chromosomes of two species ofCylindrotettix (Leptysminae, Acrididae)

Abstract

Twenty-four males ofCylindrotettix obscurus and ten ofC. santarosae (Leptysminae, Acrididae) from Rio Claro, Brazil, were cytologically analysed. Both species are 2n = 23 ♂ (22+XO), all chromosomes being telocentric except for 4 acrocentric pairs ofC. obscurus. Two different B-chromosomes were found inC. obscurus: (1) a small, mitotically unstable telocentric (B^T) and (2) a medium-sized, mitotically stable submetacentric (B¹). B¹ has a large pericentromeric heterochromatic portion and two distal euchromatic segments where both terminal associations and interstitial chiasmata are regularly formed producing a ring univalent at prophase I. B¹ is almost certainly an iso-chromosome which has undergone a centromeric shift through pericentric inversion or centric transposition (heterochromatin duplication or deletion could also be the cause of arm inequality). InC. santarosae, a single male carried a small mitotically stable telocentric B-chromosome.

Both B types ofC. obscurus significantly increased the production of abnormal sperm (diploid, tetraploid and micro-spermatids) when compared to 0B individuals although their effects were differential: B^T induced higher frequencies of microspermatids especially in those follicles with odd B^T numbers, while in B¹ carriers the increase was chiefly due to polyploid spermatids. None of the Bs ofC. obscurus affected cell mean chiasma frequency nor the between-cell variance. The B ofC. santarosae had no effect on abnormal sperm production but it probably increased cell mean chiasma frequency.