Effects of B chromosomes and supernumerary segments on morphometric traits and adult fitness components in the grasshopper, Dichroplus elongatus (Acrididae)

Linking genome size variation to population phenotypic variation within the rotifer, Brachionus asplanchnoidis

Eukaryotic organisms usually contain much more genomic DNA than expected from their biological complexity. In explaining this pattern, selection-based hypotheses suggest that genome size evolves through selection acting on correlated life history traits, implicitly assuming the existence of phenotypic effects of (extra) genomic DNA that are independent of its information content. Here, we present conclusive evidence of such phenotypic effects within a well-mixed natural population that shows heritable variation in genome size. We found that genome size is positively correlated with body size, egg size, and embryonic development time in a population of the monogonont rotifer Brachionus asplanchnoidis. The effect on embryonic development time was mediated partly by an indirect effect (via egg size), and a direct effect, the latter indicating an increased replication cost of the larger amounts of DNA during mitosis. Our results suggest that selection-based change of genome size can operate in this population, provided it is strong enough to overcome drift or mutational change of genome size.

Insights into the karyotype evolution and speciation of the beetle Euchroma gigantea (Coleoptera: Buprestidae)

Euchroma Dejean, 1833 (Buprestidae: Coleoptera) is a monotypic genus comprising the species Euchroma gigantea, with populations presenting a degree of karyotypic variation/polymorphism rarely found within a single taxonomic (specific) unit, as well as drastically incompatible meiotic configurations in populations from extremes of the species range. To better understand the complex karyotypic evolution of E. gigantea, the karyotypes of specimens from five populations in Brazil were investigated using molecular cytogenetics and phylogenetic approaches. Herein, we used FISH with histone genes as well as sequencing of the COI to determine differential distribution of markers and relationships among populations. The analyses revealed new karyotypes, with variability for chromosome number and morphology of multiple sex chromosome mechanisms, occurrence of B chromosome variants (punctiform and large ones), and high dispersion of histone genes in different karyotypes. These data indicate that chromosomal polymorphism in E. gigantea is greater than previously reported, and that the species can be a valuable model for cytogenetic studies. The COI phylogenetic and haplotype analyses highlighted the formation of three groups with chromosomally polymorphic individuals. Finally, we compared the different karyotypes and proposed a model for the chromosomal evolution of this species. The species E. gigantea includes at least three cytogenetically polymorphic lineages. Moreover, in each of these lineages, different chromosomal rearrangements have been fixed. Dispersion of repetitive sequences may have favored the high frequency of these rearrangements, which could be related to both adaptation of the species to different habitats and the speciation process.

Phenotypic Variation and Sexual Size Dimorphism in Dichroplus elongatus (Orthoptera: Acrididae)

Patterns of body size evolution are of particular interest because body size can affect virtually all the physiological and life history traits of an organism. Sexual size dimorphism (SSD), a difference in body size between males and females, is a widespread phenomenon in insects. Much of the variation in SSD is genetically based and likely due to differential selection acting on males and females. The importance of environmental variables and evolutionary processes affecting phenotypeic variation in both sexes may be useful to gain insights into insect ecology and evolution. Dichroplus elongatus Giglio-Tos is a South American grasshopper widely distributed throughout Argentina, Uruguay, most of Chile, and southern Brazil. In this study, we analyzed 122 adult females of D. elongatus collected in eight natural populations from central-east Argentina. Females show large body size variation among the analyzed populations and this variation exhibits a strong relationship with fecundity. Our results have shown that larger females were more fecund than smaller ones. We found that ovariole number varied along a latitudinal gradient, with higher ovariole numbers in populations from warmer locations. A considerable female-biased SSD was detected. SSD for three analyzed morphometric traits scaled isometrically. However, SSD for thorax length displayed a considerable variation across the studied area, indicating a larger relative increase in female size than in male size in warmer environmental conditions.

B-chromosome and V-shaped spot asymmetry in the common frog (Rana temporaria L.) populations

The common frog Rana temporaria is characterised by a large variety of colorations and dorsal patterns manifested by a V-shaped spot, which differs in the form, the number of elements and in the regularity. Some populations of R. temporaria are polymorphic for B-chromosome. V-shaped spot asymmetry and B-chromosome occurrence was studied in seven R. temporaria populations inventoried from Belarus country. The analysis of the dorsal V-shaped spot pattern asymmetry in brown frog populations implies that the persistence of additional chromosomes in populations may be one of the factors disturbing the developmental stability of some individuals. Although, Bs may confer some selective advantage to their carriers, which makes them react differently to environmental changes from do non-carriers.

Morphometric differentiation in Cornops aquaticum (Orthoptera: Acrididae): associations with sex, chromosome, and geographic conditions

The water-hyacinth grasshopper Cornops aquaticum (Bruner) (Orthoptera: Acrididae) is native to South America and inhabits lowlands from southern Mexico to central Argentina and Uruguay. This grasshopper feeds and lays eggs on species from the genera Eichhornia and Pontederia. Particularly, Eichhornia crassipes is considered "the world's worst water weed," and the release of C. aquaticum was proposed as a form of biological control. Morphometric variation on the chromosomally differentiated populations from the middle and lower Paraná River and its possible association with geographic, sex, and chromosomal conditions was analyzed. Significant phenotype variation in C. aquaticum population was detected. C. aquaticum presents body-size sexual dimorphism, females being bigger than males. Female-biased sexual size dimorphism for all five analyzed traits was detected. The assessment of variation in sexual size dimorphism for tegmen length showed that this trait scaled allometrically, indicating that males and females did not vary in a similar fashion. The detected allometry was consistent with Rensch's rule demonstrating greater evolutionary divergence in male size than in female size and suggests that males are more sensitive to environmental condition. The analysis of morphometric variation in the context of chromosome constitution showed that the presence of fusion 1/6 was related to body-size variation. Fusion carriers displayed bigger body size than standard homozygotes. Besides, a positive relationship between tegmen length and the number of fused chromosomes was detected, showing a chromosome dose effect. Because the highest frequency of fusions has been found in the lower Paraná River, a marginal environment for this species, the results found would support the hypothesis that some supergenes located in the fusions may be favored in the southern populations, thus contributing to the establishment and maintenance of the polymorphism.

Latitudinal clines in the grasshopper Dichroplus elongatus: coevolution of the A genome and B chromosomes?

Argentine populations of Dichroplus elongatus (Orthoptera: Acrididae) are polymorphic for B chromosomes. Previous studies showed that B chromosomes affect body size and some fitness components in Northwestern populations. We studied phenotype and B's variation patterns along a latitudinal cline as well as the relationship between karyotype and body size related traits in 17 populations from East. Body size related traits showed a 'saw tooth' pattern of variation being small at low and high latitudes and large at intermediate latitudes in most of the analysed populations. Analyses of variance and principal components demonstrated that in most analysed populations B carrier males are associated with a decrease in body size related traits with respect to individuals with standard karyotype. Accordingly with the relationship between karyotype and body size, an opposite pattern of latitudinal variation in the frequencies of B's with respect to body size variation was observed in this area. i.e. smaller individuals tend to have a higher frequency of B chromosomes. The comparison of the differentiation of both karyotype and body size traits with molecular neutral markers demonstrated the relative importance of selection moulding chromosome and phenotype variation. The observed pattern of phenotypic variation is likely to be the result of local adaptation to season length along the latitudinal gradient. The observed contrary pattern of B's clinal variation may reflect the population ability to maintain this chromosome in relation to the local adaptation. The available evidence indicates that the distribution of B chromosome frequency was shaped by selective factors.

Spatial genetic structure and mitochondrial DNA phylogeography of Argentinean populations of the grasshopper Dichroplus elongatus

Many grasshopper species are considered of agronomical importance because they cause damage to pastures and crops. Comprehension of pest population dynamics requires a clear understanding of the genetic diversity and spatial structure of populations. In this study we report on patterns of genetic variation in the South American grasshopper Dichroplus elongatus which is an agricultural pest of crops and forage grasses of great economic significance in Argentina. We use Direct Amplification of Minisatellite Regions (DAMD) and partial sequences of the cytochrome oxydase 1 (COI) mitochondrial gene to investigate intraspecific structure, demographic history and gene flow patterns in twenty Argentinean populations of this species belonging to different geographic and biogeographic regions. DAMD data suggest that, although genetic drift and migration occur within and between populations, measurable relatedness among neighbouring populations declines with distance and dispersal over distances greater than 200 km is not typical, whereas effective gene flow may occur for populations separated by less than 100 km. Landscape analysis was useful to detect genetic discontinuities associated with environmental heterogeneity reflecting the changing agroecosystem. The COI results indicate the existence of strong genetic differentiation between two groups of populations located at both margins of the Paraná River which became separated during climate oscillations of the Middle Pleistocene, suggesting a significant restriction in effective dispersion mediated by females and large scale geographic differentiation. The number of migrants between populations estimated through mitochondrial and DAMD markers suggest that gene flow is low prompting a non-homogeneous spatial structure and justifying the variation through space. Moreover, the genetic analysis of both markers allows us to conclude that males appear to disperse more than females, reducing the chance of the genetic loss associated with recent anthropogenic fragmentation of the D. elongatus studied range.

Selfish genetic elements and sexual selection: their impact on male fertility

Females of many species mate with more than one male (polyandry), yet the adaptive significance of polyandry is poorly understood. One hypothesis to explain the widespread occurrence of multiple mating is that it may allow females to utilize post-copulatory mechanisms to reduce the risk of fertilizing their eggs with sperm from incompatible males. Selfish genetic elements (SGEs) are ubiquitous in eukaryotes, frequent sources of reproductive incompatibilities, and associated with fitness costs. However, their impact on sexual selection is largely unexplored. In this review we examine the link between SGEs, male fertility and sperm competitive ability. We show there is widespread evidence that SGEs are associated with reduced fertility in both animals and plants, and present some recent data showing that males carrying SGEs have reduced paternity in sperm competition. We also discuss possible reasons why male gametes are particularly vulnerable to the selfish actions of SGEs. The widespread reduction in male fertility caused by SGEs implies polyandry may be a successful female strategy to bias paternity against SGE-carrying males.

Selfish genetic elements and sexual selection: their impact on male fertility

Females of many species mate with more than one male (polyandry), yet the adaptive significance of polyandry is poorly understood. One hypothesis to explain the widespread occurrence of multiple mating is that it may allow females to utilize post-copulatory mechanisms to reduce the risk of fertilizing their eggs with sperm from incompatible males. Selfish genetic elements (SGEs) are ubiquitous in eukaryotes, frequent sources of reproductive incompatibilities, and associated with fitness costs. However, their impact on sexual selection is largely unexplored. In this review we examine the link between SGEs, male fertility and sperm competitive ability. We show there is widespread evidence that SGEs are associated with reduced fertility in both animals and plants, and present some recent data showing that males carrying SGEs have reduced paternity in sperm competition. We also discuss possible reasons why male gametes are particularly vulnerable to the selfish actions of SGEs. The widespread reduction in male fertility caused by SGEs implies polyandry may be a successful female strategy to bias paternity against SGE-carrying males.