Chromosomal polymorphism, morphometric traits and mating success in Leptysma argentina Bruner (Orthoptera).

The South-American species Leptysma argentina Bruner is polymorphic for a centric fusion between chromosomal pairs 3 and 6 (fusion 3/6). Cytogenetic and morphological studies revealed that fusion 3/6 significantly increases some morphometric variables in males. A selection components study showed that the fusion is positively selected for longevity, the direct effect being exerted on thorax height. Moreover, a unisexual approach revealed that sexual male selection acts by increasing third femur length. In the present study the effect of fusion 3/6 on morphometric variables and the effect of body size and karyotype on mating success in both males and females were analysed through a bisexual approach. Total body length (TL) and third femur length (FL) were significantly larger amongst the fusion carrier females. Besides, the largest females and the carriers of the fusion have increased mating success. The selection differential suggests that directional sexual selection favours the females with increased TL and fusion dosage; however, the selection gradient revealed that none of the variables here analysed is a direct target of sexual selection. The male sexual selection study agrees with previous results, showing the importance of the third pair of legs in male mating success among grasshoppers.

Inversion polymorphisms and natural selection in Trimerotropis pallidipennis (Orthoptera).

Grasshoppers have been much less studied than Drosophila when it comes to inversion polymorphisms, despite the occurrence of this rearrangement in several species of grasshoppers. In the present study, 354 males from a natural population of the New World species Trimerotropis pallidipennis, polymorphic for 6 pericentric inversions in 4 chromosome pairs, were sampled at the beginning and at the end of the adult life span. This sampling, along with the fact that generations in this grasshopper are annual and discrete, was done to detect differential adult male longevity among karyotypes and departures from formal null models, such as gametic phase equilibrium. These methods allow the detection of natural selection taking place in the wild. The comparison between age classes showed that some inversions were significantly more frequent in one sample, thus revealing the operation of natural selection. Gametic phase disequilibrium was detected in the sample of aged males but not in the sample of young ones. Furthermore, here we aim to detect the phenotypic targets of longevity selection by examining morphometric characters, in order to have a clearer idea of the relation between inversions and natural selection in this species. These results corroborate previous studies that suggested that the inversions are involved in natural selection, and an adaptive model has been proposed for the pattern of inversion frequencies throughout several populations at different altitudes and latitudes.

Chromosome inversion polymorphisms influence morphological traits in Trimerotropis pallidipennis (Orthoptera).

Trimerotropis pallidipennis is a New World grasshopper whose South-American populations are polymorphic for six pericentric inversions. Previous work has demonstrated that the frequences of these inversions correlate with climatic variables, and hence a possible adaptive pattern was put forward. In the present work we analysed a sample of a natural population of T pallidipennis to ascertain whether the chromosomal inversions have effects on exophenotypes. Two hundred and sixty-eight males coming from a natural population at Uspallata, Mendoza Province, Argentina were analysed, and it was observed that most inversions had significant effects on phenotypes. Furthermore, some body size-related characteristics (such as tegmina length) were correlated with the number of inversions. Individuals from populations at higher altitude or latitude (i.e., at lower minimum temperatures), along with higher frequencies of standard sequences, were significantly smaller, and this coherence between interpopulational with intrapopulational results may indicate that the diminished body size of the standard sequence-carrying individuals may be caused by an effect of the inversions, or genes within the inversions, on body size. We finally put forward the hypothesis that reduced body size in a context of reduced minimum temperature may be a response to shortened development season, and so smaller individuals may be advantageous.

Chromosomal polymorphism, morphological traits and male mating success in Leptysma argentina (Orthoptera).

Leptysma argentina (Acrididae: Orthoptera) is polymorphic for a centric fusion between pairs 3 and 6 of its basic chromosomal complement. With the aim of carrying out an analysis of selection components in this species, male mating success, in relation to karyotype and three morphometric traits, was assessed by using experimental contests with four males and one female per mating cage. Karyotype was assessed in terms of fused chromosome 3/6 dosage and coded as "0", "1" and "2" for unfused homozygotes, heterozygotes and fusion homozygotes, respectively. The individual ANOVA showed significant differences between "successful" and "unsuccessful" males in all four measured characters. Fusion carriers showed a higher relative fitness. Since the phenotypic correlations among traits were significant, the selective effects were assessed by means of a multivariate analysis, which demonstrated that selection acted significantly on femur length only, selection on karyotype being indirect. Given that the fusion frequency has been constant since 1985, the presence of selective trade-offs cannot be ruled out.

Chromosome polymorphisms and natural selection in Leptysma argentina (Orthoptera). IV. Survival selection acts on karyotype polymorphisms at the adult stage and before.

Deviation from formal null models and the comparisons between age classes are amongst the most direct methods of demonstrating ongoing natural selection. We have sampled 207 males from a population of the grasshopper Leptysma argentina at the onset and the end of the adult life span for this species; generations are synchronical and do not overlap. The karyotypic frequencies for a fusion (F) and a supernumerary segment (s1) were examined. The young male sample was heterogeneous and did not fit to the Hardy-Weinberg proportions, whereas the aged male sample did not deviate from H-W expectations in both analyses. Likewise, the young male sample showed fitness heterosis for the s1 carriers that are fusion homozygotes. We interpreted the significant results of karyotype frequency in both samples as the result of survival selection. Evidently natural selection acts on male adult viability as well as on other fitness components prior to adult survival, thus changing karyotype frequencies between generations. Likewise, we also detected a strong interaction between polymorphisms, whereas heterosis for s1 heterozygotes might be contributing to the maintenance of the s1 polymorphism. The actual target of selection remains unidentified.

Chiasma interference is blind to centromeres.

It has long been assumed, since the early works of Mather, that the centromere plays a central role in chiasma position determination, so much so that in all sequential models chiasma determination was supposed to start or finish at this point. More specifically, it has been assumed that the centromere acts as a barrier to the transmission of interference, so that a chiasma in the vicinity of a centromere would not affect the probability of chiasma formation across at this point. Some statistical analyses seemed to ratify this supposition. However, a reassessment of the literature led us to the conclusion that the statistical analyses that were not flawed were consistent in showing that interference may act across the centromere. Using large sets of chiasma data from the grasshoppers Leptysma argentina and Chorthippus brunneus and applying statistical approaches that involved either the calculation of coincidence or correlating the distances between the centromere and the nearest chiasma in either arm, it is concluded: 1. that interference acts across the centromere; 2. that the action of interference is not changed by the presence of an intervening centromere.

Inversion polymorphisms in Trimerotropis pallidipennis (Orthoptera): clinical variation along an altitudinal gradient.

Six populations of Trimerotropis pallidipennis located along an altitudinal gradient in the Antinaco-Los Colorados valley (La Rioja, Argentina), were cytologically analysed. The male karyotype consists of 23 chromosomes (22 + XO) with three long pairs submetacentric, the X-chromosome metacentric and the remaining ones basically acrocentric. Populations from La Rioja were polymorphic for seven pericentric inversions. Correlations between chromosome frequencies and altitude were statistically tested. In most cases, variables were either positively or negatively related in a significant fashion. As a consequence, mean populational heterozygosis (measured as the mean number of heteromorphic bivalents per individual per population) was also positively correlated with altitude. These results suggest that altitude (or other correlated environmental variable) may exert a differential selective pressure on coadapted gene blocks in the mutually inverted sequences. The possible relation of these results with the central-marginal model is discussed.