Against the central-marginal model: Three cases in chromosomally polymorphic grasshoppers (Orthoptera: Acrididae)

Abstract

Abstract. The central-marginal model is widely accepted in chromosomally polymorphic species of Drosophila. In fact, geographically and ecologically central populations of Drosophila show higher levels of polymorphism for paracentric inversions, whereas marginal populations tend to be monomorphic. This fact has been variously explained. Chromosomal polymorphisms in grasshoppers have also been attributed to show such geographical structuring, as in the case of the South-American grasshopper Dichroplus pratensis Bruner (Orthoptera: Acrididae). However, in three other cases involving Acrididae -Leptysma argentina Bruner, Trimerotropis pallidipennis (Burmeister) and Cornops aquaticum (Bruner), it is clear that chromosomal polymorphisms (sometimes with a wide extension over the Argentine area) do not conform to this pattern, and show instead clear correlations with environmental variables, especially minimum temperature, showing low or null frequencies of the rearrangements at one extreme of the environmental gradient and with high or fixed frequencies at the other. Furthermore, this correlation with temperature might also be true in the case of D. pratensis. These aforementioned examples emphasise the dangers of over-generalization when discussing chromosomal polymorphisms, and suggests that such polymorphisms should be considered very much in a case-specific manner in terms of the particular genetic system under study. 317* Affiliated to CONICET (Consejo Nacional de Investigaciones Científicas y Técnicas).

Chromosome fusion polymorphisms in the grasshopper, Dichroplus fuscus (Orthoptera: Acrididae: Melanoplinae): Insights on meiotic effects

“…The best studied species to date is a congener of D. fuscus, D. pratensis in which several fusion polymorphisms were studied in most of its wide geographic range showing a clear central-marginal pattern: number and frequency of fusions is high in central (ecologically optimal) areas and decreases clinally to zero in marginal environments (Bidau & Martí, 2002). The former scenario was challenged by Colombo (2012) on the basis of two other cases of fusion polymorphisms in the Acrididae, suggesting, with little proof other than geographic distribution, that observed clinal variation of fusions does not necessarily reflect a central-marginal pattern. However, as Bidau & Martí (2013) replied, the species cited by Colombo's (2012) as examples are known chromosomally only from a small marginal fraction of their large distribution ranges.…”

The central-marginal hypothesis in acridid Orthoptera: A critique of Colombo's (2012) article

“…In a recent paper, Colombo (2012) discussed the application of the central-marginal hypothesis using as model organisms three species of South American grasshoppers that exhibit chromosomal polymorphisms (Robertsonian translocations and pericentric inversions) which he also compared to our own data on the Neotropical species Dichroplus pratensis Bruner (Acrididae: Melanoplinae). This species shows a clear central-marginal pattern, not only with regard to chromosomal polymorphisms (Robertsonian translocations and B chromosomes) but also many other life-history characteristics (Bidau & Martí, 2002;Bidau et al, 2012).…”

“…All the grasshopper species (Orthoptera: Acrididae) used by Colombo (2012) as examples for challenging the reality of the central-marginal pattern have vast geographic distributions (Eades et al, 2012). For example, the geographic distribution of Cornops aquaticum (Bruner) is millions of square km, from the borders of USA and Mexico, throughout Central America and most of South America reaching almost the north of the Argentine Patagonia (Adis et al, 2007).…”

“…These chromosomal clines obviously coincide with a number of environmental clines, since climatic factors vary gradually towards the margins of the distribution range involving latitude, longitude, and elevation. All these factors were not taken into account by Colombo (2012) in his revision of the literature pertaining to D. pratensis, such that his review undoubtedly missed very important and pertinent publications. Additionally, Colombo's (2012) stress on temperature does not seem warranted according to the available published information: many abiotic factors correlated with marginality may affect characteristics of populations.…”

“…Moreover, of the species used by Colombo (2012) as examples of his general thesis, one cannot be sure what kind of populations the author actually analysed. In the cases of C. aquaticum and L. argentina, they are undoubtedly geographically peripheral in terms of distribution.…”