Dardo A. Martí

Clinal Variation of Body Size in Dichroplus pratensis (Orthoptera: Acrididae): Inversion of Bergmann's and Rensch's Rules

Abstract We studied geographical body size variation in 23 populations and sexual size dimorphism in 19 populations of the grasshopper Dichroplus pratensis Bruner in Argentina, spanning 22° of latitude and between sea level and >2,474-m altitude. Six characters were measured in 609 individuals (334 males and 275 females): total body, femur 3, tibia 3, tegmina and pronotum lengths, and pronotum height. Significant negative correlations between all six characters and latitude occurred. Correlations with altitude were weaker, although individuals at higher altitudes tended to be smaller, except at high latitudes. Extreme latitudes and altitudes correspond to the geographical and ecological margins of the species distribution and share comparable strenuous environmental conditions. Thus, this species follows the converse to Bergmann’s rule, that is, individuals tend to be smaller as the environment becomes cooler (when latitude or altitude are used as a proxy for temperature). When the six morphological characters where analyzed in both sexes with respect to mean January and July temperatures at each locality, positive (although not significant) correlations were obtained in all cases. Sexual dimorphism occurred throughout the geographic range involving not only differences in body size but also in body proportions, which can be explained through differential allometric growth and shorter developmental time in males (the species exhibits protandry). The latter can also explain the inversion of Rensch’s rule that occurs in D. pratensis: sexual size dimorphism decreases as general body size increases.

Tracking the evolution of sex chromosome systems in Melanoplinae grasshoppers through chromosomal mapping of repetitive DNA sequences

BackgroundThe accumulation of repetitive DNA during sex chromosome differentiation is a common feature of many eukaryotes and becomes more evident after recombination has been restricted or abolished. The accumulated repetitive sequences include multigene families, microsatellites, satellite DNAs and mobile elements, all of which are important for the structural remodeling of heterochromatin. In grasshoppers, derived sex chromosome systems, such as neo-XY♂/XX♀ and neo-X1X2Y♂/X1X1X2X2♀, are frequently observed in the Melanoplinae subfamily. However, no studies concerning the evolution of sex chromosomes in Melanoplinae have addressed the role of the repetitive DNA sequences. To further investigate the evolution of sex chromosomes in grasshoppers, we used classical cytogenetic and FISH analyses to examine the repetitive DNA sequences in six phylogenetically related Melanoplinae species with X0♂/XX♀, neo-XY♂/XX♀ and neo-X1X2Y♂/X1X1X2X2♀ sex chromosome systems.ResultsOur data indicate a non-spreading of heterochromatic blocks and pool of repetitive DNAs (C0t-1 DNA) in the sex chromosomes; however, the spreading of multigene families among the neo-sex chromosomes of Eurotettix and Dichromatos was remarkable, particularly for 5S rDNA. In autosomes, FISH mapping of multigene families revealed distinct patterns of chromosomal organization at the intra- and intergenomic levels.ConclusionsThese results suggest a common origin and subsequent differential accumulation of repetitive DNAs in the sex chromosomes of Dichromatos and an independent origin of the sex chromosomes of the neo-XY and neo-X1X2Y systems. Our data indicate a possible role for repetitive DNAs in the diversification of sex chromosome systems in grasshoppers.

Neo-sex chromosome diversity in Neotropical melanopline grasshoppers (Melanoplinae, Acrididae).

We report the results of a study on the neo-sex chromosome systems of six Neotropical Melanoplinae species for contributing to a better understanding of their origin and behaviour of these systems. Our analyses included detailed descriptions of the structure and behaviour of the sex chromosome configurations in male and female meiosis of species belonging to the genera Ronderosia, Dichromatos and Atrachelacris. Three species, R. forcipatus, R. malloi and A. unicolor, showed typical Robertsonian fusion-derived neo sex-chromosomes. However, the male metaphase I orientation of R. bergi sex pair indicated that more than one rearrangement was involved in its origin. The two species of Dichromatos presented a multiple neo-X1X2Y/X1X1X2X2 sex system, with two Robertsonian fusions involved in their genesis. Observations of female meiosis, confirmed the nature of the sex-chromosomes analyzed. Our results also showed different degrees of homology divergence between the neo-sex chromosomes and emphasize the plasticity of the chromosome complement of the Neotropical Melanoplinae to establish Robertsonian fusions and generate novel sex-chromosome systems. We also discuss karyotypic diversity within this group in terms of the centromeric drive theory of chromosomal evolution.

Bergmann's rule across the equator: a case study in Cerdocyon thous (Canidae)

1. The variation in cranial size of the crab-eating fox Cerdocyon thous was analysed in relation to latitude and several environmental variables throughout its distribution in South America. 2. We tested the existence of clines to determine whether this canid follows Bergmanns rule to the north and south of the Equator. Also, using niche modelling, we analysed whether the climatic changes during the last glaciation could have influenced Bergmanns rule in this species. We quantified the size of the cranium of C. thous (n = 300). The data were divided into two groups: (i) south of the Equator (n = 163) and (ii) north of the Equator (n = 137). We performed correlations, OLS regressions and simultaneous autoregressions to analyse the relationship between the variation in size and different geographic and environmental variables. Data of occurrence (n = 594) together with ambient variables from the present and the last glacial maximum (LGM) were used to predict the occurrence of C. thous with the implementation of the maximum entropy method. Present-day and historical distribution maps were obtained. 3. The variation in the size of the cranium of C. thous showed two trends. In the south of Equator, we observed that the size of the skull shows an inverse relationship with temperature-related variables and a positive one with precipitation, while in north of the Equator, we observed the opposite relationship. Populations south of the Equator follow Bergmanns rule showing increasing size with increasing latitude. To the north of the Equator, a non-Bergmannian pattern occurs because size decreases with increasing latitude. 4. Niche modelling showed two present-day groupings in South America, one north of Amazonia and the other south. However, for the period of the LGM, four groups emerged, possibly related to the four subspecies presently described for C. thous. Therefore, it is possible that the observed pattern - southern populations following Bergmanns rule while northern populations reflecting the opposite - has been influenced by the events that occurred during the LGM that could have led to the differentiation of populations.

Meiosis and the Neo-XY system of Dichroplus vittatus (Melanoplinae, Acrididae): a comparison between sexes.

The origin of neo-XY sex systems in Acrididae is usually explained through an X-autosome centric fusion, and the behaviour of the neo-sex chromosomes has been solely studied in males. In this paper we analysed male and female Dichroplus vittatus. The karyotype comprises 2n = 20 chromosomes including 9 pairs of autosomes and a sex chromosome pair that includes a large metacentric neo-X and a small telocentric neo-Y. We compared the meiotic behaviour of the sex bivalent between both sexes. Mean cell autosomal chiasma frequency was low in both sexes and slightly but significantly higher in males than in females. Chiasma frequency of females increased significantly when the sex-bivalent was included. Chiasma distribution was basically distal in both sexes. Behaviour of the neo-XY pair is complex as a priori suggested by its structure, which was analysed in mitosis and meiosis of diploid and polyploid cells. During meiosis, orientation of the neo-XY is highly irregular; only 21% of the metaphase I spermatocytes show standard orientation. In the rest of cells, the alternate or simultaneous activity of an extra kinetochore in the distal end of the short arm (XL) of the neo-X, determined unusual MI orientations and a high frequency of non-disjunction and lagging of the sex-chromosomes. In females, the neo-XX bivalent had a more regular behaviour but showed 17% asynapsis in the XL arm which, in those cases orientated its distal ends towards opposite spindle poles suggesting, again, the activity of a second kinetochore. The dicentric nature and the unstable meiotic behaviour of the sex neo-chromosomes of D. vittatus suggest a recent origin of the sex determination mechanism, with presumable adaptive advantages which could compensate their potential negative heterosis. Our observations suggest that the origin of the neo-sex system was a tandem fusion of two original telocentric X-chromosomes followed by another tandem fusion with the small megameric bivalent and a further pericentric inversion of the neo-X. The remaining autosomal homolog resulted in the neo-Y chromosome.

Contrasting patterns of sexual size dimorphism in the grasshoppers Dichroplus vittatus and D. pratensis (Acrididae, Melanoplinae)*

Abstract Sexual size dimorphism (SSD) can be the result of sexual selection (SS) or natural selection (NS). Due to male-male competition for access to females, SS could favor an increase in male body size. On the other hand, larger size in females could be favored by NS, since egg production is directly correlated with body size. Rensch`s rule states that SSD increases with increasing body size in animals, where males are the larger sex, and decreases when females are larger than males. Thus, Renschs rule predicts that in those insects where females are larger than males, SSD should decrease with increasing body size, when comparing populations and species. We analyzed SSD in 19 Argentine populations of the grasshoppers Dichroplus vittatus and 25 of D. pratensis. Both species show latitudinal and altitudinal variation in body size, following the converse to Bergmanns rule: body size decreases with increasing latitude and decreasing ambient temperature. SSD occurs in both species across their geographical distribution ranges, also involving differences in allometry and shorter developmental times in males. In D. vittatus, the degree of SSD increased significantly with general body size, whereas in D. pratensis SSD decreased as body size increased. A plausible explanation of SSD is that SS favors a differential increase in female body size related to a preference by males for more fecund females. Given the close phylogenetic relationship between both species, the differences in SSD between them may be the result of differential natural and sexual selective pressures. In D. vittatus both sexes could be reacting differently to environmental conditions regarding body size, while in D. pratensis protandry could be the main factor behind SSD.

Neo-sex chromosomes of Ronderosia bergi: insight into the evolution of sex chromosomes in grasshoppers

Sex chromosomes have evolved many times from morphologically identical autosome pairs, most often presenting several recombination suppression events, followed by accumulation of repetitive DNA sequences. In Orthoptera, most species have an X0♂ sex chromosome system. However, in the subfamily Melanoplinae, derived variants of neo-sex chromosomes (neo-XY♂ or neo-X1X2Y♂) emerged several times. Here, we examined the differentiation of neo-sex chromosomes in a Melanoplinae species with a neo-XY♂/XX♀ system, Ronderosia bergi, using several approaches: (i) classical cytogenetic analysis, (ii) mapping via fluorescent in situ hybridization of some selected repetitive DNA sequences and microdissected sex chromosomes, and (iii) immunolocalization of distinct histone modifications. The microdissected sex chromosomes were also used as sources for Polymerase chain reaction (PCR) amplification of RNA-coding multigene families, to study variants related to the sex chromosomes. Our data suggest that the R. bergi neo-Y has become differentiated after its formation by a Robertsonian translocation and inversions, and has accumulated repetitive DNA sequences. Interestingly, the ex autosomes incorporated into the neo-sex chromosomes retain some autosomal post-translational histone modifications, at least in metaphase I, suggesting that the establishment of functional modifications in neo-sex chromosomes is slower than their sequence differentiation.

A test of Allen's rule in ectotherms: the case of two south American Melanopline Grasshoppers (Orthoptera: Acrididae) with partially overlapping geographic ranges

We studied the geographic variation of three morphometric characters in relation to body size in two South American grasshoppers (Acrididae), Dichroplus vittatus Bruner and D. pratensis Bruner to test Allens rule in these ectotherms. Since both species follow the converse to Bergmanns rule owing to latitudinal and/or altitudinal variation in time available for growth and reproduction, geographic variation in body size proportions of protruding parts may obey to differential allometric growth in different geographic areas. Alternatively, it could reflect true Allenian variation related to thermoregulation. Body proportions were studied by correlation/regression analyses with geographic and climatic variables. In D. pratensis, body proportions increased with latitude and decreased with altitude. These results probably obey to the effects of water balance and seasonality on final body size, and on the allometric growth of the three studied characters not being related to thermoregulation. In D. vittatus, a generally non-significant trend towards the decrease of the mean proportions of all three characters with increasing latitude was observed. Nevertheless, also in this species, it is probable that the environmental gradient responds to seasonality factors (although not to water balance) that affect the length of growing season and, in consequence, body size and its allometric relationships. We conclude that the regularities in the geographic distribution of body proportions of D. pratensis and D. vittatus do not follow Allens rule in the sense of thermoregulation, and result from variables that determine growing season length and the allometric growth of different body parts.

Geographic and climatic factors related to a body-size cline in Dichroplus pratensis Bruner, 1900 (Acrididae, Melanoplinae)*

Abstract We studied geographic body-size variation in males and females of 25 populations of the South American melanopline grasshopper Dichroplus pratensis Bruner, 1900, along more than 22 degrees of latitude (S) and between 0 and almost 2500 m of altitude. Using mean body length of each sex and factors obtained from PCA analyses of six morphometric linear characters, it was shown that D. pratensis follows the converse to Bergmanns rule, becoming smaller at higher latitudes and altitudes. Variability of body size increased with latitude and altitude in males and females. Body-size trends were statistically significantly correlated with ambient temperature (annual mean, minimum and maximum), precipitation (annual mean, minimum and maximum), and two estimators of seasonality, the difference between the maximum and minimum temperatures, and the difference between maximum and minimum precipitation for each locality; both nonparametric correlations were positive. Body size was also positively and significantly correlated with Actual Evapotranspiration (AET), a measure of primary productivity, and with Potential Evapotranspiration (PET), a measure of ambient energy, but not with water balance (WB). Some allometric relationships also showed geographic variation. We suggest that the observed decrease in size with latitude and the increase in morphological variability, are joint consequences of the shortening of the growing season towards the south, the increasing seasonality and climatic unpredictability, lower primary productivity (as represented by AET); and that the species exhibits protandry, which contributes in the south, to smaller and more variably sized males, and smaller but more constant body size in females.

Effects of Abiotic Factors on the Geographic Distribution of Body Size Variation and Chromosomal Polymorphisms in Two Neotropical Grasshopper Species (Dichroplus: Melanoplinae: Acrididae)

We review the effects of abiotic factors on body size in two grasshopper species with large geographical distributions: Dichroplus pratensis and D. vittatus, inhabiting Argentina in diverse natural habitats. Geographical spans for both species provide an opportunity to study the effects of changes in abiotic factors on body size. The analyses of body size distribution in both species revealed a converse Bergmannian pattern: body size is positively correlated with latitude, altitude, and seasonality that influences time available for development and growth. Allen’s rule is also inverted. Morphological variability increases towards the ends of the Bergmannian clines and, in D. pratensis, is related with a central-marginal distribution of chromosomal variants that influence recombination. The converse Bergmannian patterns influence sexual size dimorphism in both species but in different fashions. Body size variation at a microspatial scale in D. pratensis is extremely sensitive to microclimatic clines. We finally compare our results with those for other Orthopteran species.