Population cytology of British grasshoppers. II. Annual variation in chiasma frequency

Geographical variability in the pericentric inversion system of the grasshopper Trimerotropis pseudofasciata

Island and mainland populations of Trimerotropis pseudofasciata from California were compared with respect to the nature and extent of their percentric inversion systems. Island populations generally have more chromosomes polymorphic for centromere position than mainland populations and a considerably higher percentage of the genome in these island populations is in a structurally heterozygous state. Thus, although geographically peripheral, the islands provide habitats capable of supporting denser and more chromosomally polymorphic populations than the mainland. Chiasmata are generally localized to terminal positions in all classes of chromosomes and do not occur in the inverted regions of inversion heterozygotes. Chiasma frequency is highest in inversion homozygotes. It is hypothesized that the inversion system in T. pseudofasciata serves the dual synergistic function of preserving allelic sequences in the inversion region intact through inversion heterozygosity and limiting the generation of variability in regions outside the inversion by increasing terminal chiasmata. Additionally, it is argued that it is the gene sequence on only the inversion chromosome that is important in Trimerotropis. This condition contrasts with the "co-adapted" pattern seen in Drosophila where the gene sequences on both chromosomes in the inversion heterozygote are simultaneously important.

MEIOSIS IN DIABROTICA (COLEOPTERA: CHRYSOMELIDAE): CHIASMA FREQUENCY AND VARIATION

Fifteen species of Diabrotica have been examined cytologically. Except for the variable presence of supernumeraries in three of the species, the karyotype is uniform throughout, with 18 meta- or submetacentric autosomes and an XX ♂: XO ♀ sex-determining mechanism. Chiasma frequency is quite variable; in addition to the basic nine per cell, formation of a second chiasma (in the other arm) results in ring frequencies ranging from zero to nine per cell. Analysis of chiasma frequency and distribution at diakinesis and metaphase reveals that the rings are bi-chiasmate and not the result of ectopic pairing. With this in mind, chiasma formation is discussed in terms of interference. Intra-arm interference is complete; the upper limit of chiasmata per arm is invariably one. The strength of interference across the centromere determines the probability of formation of a chiasma in the other arm. When interference is strong, a second chiasma is rarely formed; as interference weakens, a chiasma is more easily formed in the other arm, resulting in a higher chiasma frequency. Associated with decreasing interference and an elevated chiasma frequency is an increase in cell variance; chiasma formation is less rigorously controlled, and variability in the number of rings per cell increases. The variability of ring formation in the genus militates against distinguishing taxa on this basis. However, the importance of increased chiasma frequency (coupled as it is with greater variance) resides in its influence on the genetic plasticity and adaptive potential of the species.