Cis- and trans-acting genetic factors contribute to heterogeneity in the rate of crossing over between the Drosophila simulans clade species.

In the genus Drosophila, variation in recombination rates has been found within and between species. Genetic variation for both cis- and trans-acting factors has been shown to affect recombination rates within species, but little is known about the genetic factors that affect differences between species. Here, we estimate rates of crossing over for seven segments that tile across the euchromatic length of the X chromosome in the genetic backgrounds of three closely related Drosophila species. We first generated a set of Drosophila mauritiana lines each having two semidominant visible markers on the X chromosome and then introgressed these doubly marked segments into the genetic backgrounds of its sibling species, Drosophila simulans and Drosophila sechellia. Using these 21 lines (seven segments, three genetic backgrounds), we tested whether recombination rates within the doubly marked intervals differed depending on genetic background. We find significant heterogeneity among intervals and among species backgrounds. Our results suggest that a combination of both cis- and trans-acting factors have evolved among the three D. simulans clade species and interact to affect recombination rate.

Nucleolus organizing regions and semi-persistent nucleolus during meiosis in Spartocera fusca (Thunberg) (Coreidae, Heteroptera).

The Coreidae are cytogenetically characterized by possessing holokinetic chromosomes and a pre-reductional type of meiosis. The modal diploid chromosome number of the family is 21, with a pair of m chromosomes and an XO/XX sex chromosome determining system. Spartocera fusca presents 2n=23/24=20+2m+XO/20+2m+XX (male/female). Meiosis follows the general pattern described for heteropterans, with a diffuse stage after pachytene and a particular chromosome arrangement at both metaphase plates. S. fusca presents some cytogenetic peculiarities: the X chromosome shows a secondary constriction in a medial position, which is not a nucleolus organizing region. It has been revealed by in situ hybridization with a rDNA probe that the NOR is localized at the telomeric region of one autosomal pair. Furthermore, during the meiosis of three specimens of S. fusca a semi-persistent nucleolus was detected from early meiotic prophase until telophase II; the presence of this semi-persistent nucleolus together with the long diffuse stage detected in the specimens suggest that a continuous biosynthetic activity is required for spermiogenesis. These observations could be related to differences in the environmental, and therefore, physiological conditions of the analyzed individuals.