REPRODUCTIVE ISOLATION IN RELATION TO ALLOZYMIC AND CHROMOSOMAL DIFFERENTIATION IN THE GRASSHOPPER CALEDIA CAPTIVA

ASSOCIATION BETWEEN LATITUDINAL VARIATION FOR EMBRYONIC DEVELOPMENT TIME AND CHROMOSOME STRUCTURE IN THE GRASSHOPPER CALEDIA CAPTIVA (ORTHOPTERA: ACRIDIDAE)

From southeastern Queensland to southern Victoria, over a transect of 11° latitude, the Moreton taxon of the Australian grasshopper Caledia captiva exhibits a cline in chromosome structure that involves change from a metacentric to an acrocentric genome. In this study, we show that embryonic development time covaries with chromosome structure along the transect. Both development time and chromosome short arm length exhibit an overall negative correlation with latitude, but with maxima just south of the northern limit of the taxon's distribution. Selection for such a pattern appears to arise from changes in voltinism along the cline in season length that exists along the transect. Populations with the highest temperature thresholds for avoidance of embryonic diapause also have the slowest development time and probably represent the northern extreme of a primarily univoltine life cycle. North of this region bivoltinism increases in frequency and, as expected from a split of the season length, development time decreases. Maximum chromosome short arm length occurs in the vicinity of the northern univoltine populations, rather than at the limit of distribution where bivoltinism prevails. We conclude that variation in chromosome structure could be contributing to the heritable variation for development time that forms the basis for adaptive change in this trait. These results provide justification for investigating causal relationships between chromosome structure and development time, with an ultimate aim of understanding the adaptive significance of chromosomal variation in C. captiva.

Hierarchical structure of genetic distances: Effects of matrix size, spatial distribution and correlation structure among gene frequencies

Geographic structure of genetic distances among local populations within species, based on allozyme data, has usually been evaluated by estimating genetic distances clustered with hierarchical algorithms, such as the unweighted pair-group method by arithmetic averages (UPGMA). The distortion produced in the clustering process is estimated by the cophenetic correlation coefficient. This hierarchical approach, however, can fail to produce an accurate representation of genetic distances among populations in a low dimensional space, especially when continuous (clinal) or reticulate patterns of variation exist. In the present study, we analyzed 50 genetic distance matrices from the literature, for animal taxa ranging from Platyhelminthes to Mammalia, in order to determine in which situations the UPGMA is useful to understand patterns of genetic variation among populations. The cophenetic correlation coefficients, derived from UPGMA based on three types of genetic distance coefficients, were correlated with other parameters of each matrix, including number of populations, loci, alleles, maximum geographic distance among populations, relative magnitude of the first eigenvalue of covariance matrix among alleles and logarithm of body size. Most cophenetic correlations were higher than 0.80, and the highest values appeared for Nei's and Rogers' genetic distances. The relationship between cophenetic correlation coefficients and the other parameters analyzed was defined by an "envelope space", forming triangles in which higher values of cophenetic correlations are found for higher values in the parameters, though low values do not necessarily correspond to high cophenetic correlations. We concluded that UPGMA is useful to describe genetic distances based on large distance matrices (both in terms of elevated number of populations or alleles), when dimensionality of the system is low (matrices with large first eigenvalues) or when local populations are separated by large geographical distances.

Genetics and polymorphism of a duplicated malate dehydrogenase (MDH) locus in the grasshopper Oxya japonica japonica (Orthoptera:Acrididae:Oxyinae)

A biochemical genetic study of the enzyme malate dehydrogenase (MDH) was conducted in the grasshopper Oxya j. japonica. Analysis of MDH electrophoretic variation in this species of grasshopper shows that one of the two autosomal loci for MDH in grasshoppers, the Mdh-2 locus, controlling the anodal set of MDH isozymes, is duplicated. Results of breeding studies confirm this and the observed polymorphism at the Mdh-2 locus in the two populations of Oxya j. japonica studied can be attributed to three forms of linked alleles at the duplicated locus in equilibrium in both populations. In this respect, all individuals of this species possess heterozygous allelic combinations at the duplicated Mdh-2 locus, which may account for the spread of the duplicated locus in the populations of this species of grasshopper.

Observed relationships between protein heterozygosity and protein genetic distance and comparisons with neutral expectations

Relationships between protein genetic distance (D) and protein heterozygosity (H) were studied using allele frequency data for 42 proteins derived from multilocus electrophoretic surveys of genetic variation in over 200 invertebrate and over 300 vertebrate species.D¯andH¯values for the different proteins (mostly enzymes) were calculated, and large and significant correlations betweenD¯andH¯were found in comparisons of both intraspecific and interspecific populations. Empirical relationships betweenD¯andH¯were compared with neutral expectations under the stepwise model of neutral mutation with the assumption that populations are in equilibrium with respect to the effects of mutation and genetic drift.

At low divergence levels, a linear relationship ofD¯onH¯was observed, but at high levels of divergenceDtended towards an asymptote at highH¯. The results at high divergence cannot be explained using the approximate relationshipD=2ut(whereu= mutation rate,t= time). However, computer simulations of neutral models showed that changes of this nature in the relationship betweenD¯andH¯were to be expected as divergence increases, the equationD=2utbeing a poor approximation at highDWe therefore conclude that the observed relationships betweenD¯andH¯are, in fact, compatible with equilibrium neutral theory.

Increased chromosomal mutation rate after hybridization between two subspecies of grasshoppers

Hybridization between two chromosomally distinct subspecies of the grasshopper Caledia captiva results in a high incidence of novel chromosomal rearrangements among the backcross progeny. Rearrangements are restricted to those chromosomes derived from the F1 hybrid parent. Chromosomal involvement is nonrandom with the same rearrangement occurring repeatedly in different backcrosses. A single individual can also generate an array of different rearrangements among its offspring. Several of the rearrangements have also been found in natural populations. The nonrandom and recurrent nature of these chromosomal mutations at high frequencies provides a plausible explanation for the establishment and fixation of chromosomal rearrangements in natural populations.