Chromosomal Taxa Research Articles

Cryptic speciation in the Triatoma sordida subcomplex (Hemiptera, Reduviidae) revealed by chromosomal markers.

BackgroundChagas disease vectors (Hemiptera-Reduviidae) comprise more than 140 blood-sucking insect species of the Triatominae subfamily. The largest genus is Triatoma, subdivided in several complexes and subcomplexes according to morphology, ecology and genetic features. One of them is the sordida subcomplex, involving four species: Triatoma sordida, T. guasayana, T. garciabesi and T. patagonica. Given the great morphological similarity of these species, their taxonomic identification, evolutionary relationships and population differentiation have been controversial for many years and even today remain under discussion.MethodsWe simultaneously analyzed two chromosomal markers, C-heterochromatin distribution and 45S ribosomal genes chromosomal position, of 139 specimens from several sordida subcomplex populations from Argentina, Bolivia, Brazil and Paraguay, collected both in nature and from several established insectaries. Our results were compared with COI sequences deposited in GenBank.ResultsWe recognized five chromosomal taxa with putative hybrids, which each differ in at least one chromosome marker. Most of them present significant differences in their mtDNA sequences.ConclusionThe chromosomal taxa here show a significant chromosome differentiation involving changes in the C-heterochromatin content and in the ribosomal clusters position. This paper identifies several erroneously classified populations by morphological methods, delimits the geographical distribution of each taxon and proposes the existence of a new cryptic species, widely distributed in Argentina. We also suggest that sordida sibling species involve closely related as well as evolutionary distant species. Taxonomic status of each chromosomal taxon is discussed considering phenotypic and genetic results previously published.

Genotypic variation in progeny of the agamic grass complexPennisetum sectionBrevivalvula in West Africa

The genotypic variation of 1180 progeny from 118 genitors belonging to five taxa ofPennisetum sect.Brevivalvula has been estimated by isoenzyme electrophoresis with observations on five enzymatic systems, in order to compare the type of reproduction in polyploid and diploid taxa. A total of 112 different isozyme genotypes has been found, over all taxa. Genotypic variation was found among all progeny of the diploid populations ofP. polystachion andP. subangustum, as a consequence of their sexual reproduction system. At the polyploid level the type of reproduction appears to be predominantly agamic, but genotypic variation in the progeny was not rare: five tetraploid and one hexaploidP. pedicellatum, one pentaploid and one hexaploidP. polystachion and one hexaploidP. hordeoides, in a total of 90 genitors. Genetic relationships have been observed between the diploid sexualP. polystachion andP. subangustum, and, to a lesser extent, with the tetraploids of the same taxa as well. TetraploidP. polystachion andP. pedicellatum share genotypes with most other chromosomal taxa.

THE EFFECT OF A PARTIAL BARRIER ON THE MOVEMENT OF A HYBRID ZONE.

Computer simulations of clines (Brues, 1972; Endler, 1977) as well as theoretical arguments (Nagylaki, 1975), have shown that steps in gene frequencies will be pulled to partial barriers (areas of reduced gene flow) if they form within approximately a cline width of the partial barrier. The behavior of a hybrid zone between two chromosomal taxa ("Moreton" and "Torresian") of the acridine grasshopper Caledia captiva in southeast Queensland has been analyzed and found to comform qualitatively with a model of altered gene flow patterns. Clines in four enzyme systems were analyzed for 1983 and 1986 along a transect across the hybrid zone. The clines have shifted towards an area of regenerating forest, while homozygote frequencies have increased at this point. This forest barrier has broken the continuity of the spatial distribution of C. captiva, forming population islands in part of the hybrid zone, and thus reducing the amount of gene flow. The distance between the barrier and the original cline is approximately of the order of a cline width, so that they would be expected to interact. Historical information suggests that the secondary contact between the "Moreton" and "Torresian" taxa occurred very recently (1844-1940), due to the intensive land-clearing activities during the European settlement.

The spatial distribution of chromosomes in metaphase neuroblast cells from subspecific F1 hybrids of the grasshopper Caledia captiva

The spatial distribution of chromosomes has been analysed in radial metaphase neuroblast cells in F1 hybrid embryos generated by crossing individuals of the Moreton and Torresian (TT) chromosomal taxa of the grasshopper Caledia captiva. The Moreton individuals were of two kinds depending on whether they carried an acrocentric X (MAX) or a metacentric X (MMX). No significant associations were detected between any pair of homologous chromosomes in either male or female (MAX x TT) and (MMX x TT) F1 hybrids. This result was supported by data which showed that the mean separation between homologues is greater, although not significantly so, than the mean separation between non-homologous chromosomes within the two Moreton genomes. Indeed, in a number of cases, genome separation was clearly observed in radial metaphase preparations from these F1 hybrids. By comparison the analysis of pairwise associations between non-homologous chromosomes within the MMX and MAX Moreton genomes revealed a number of significant associations and dissociations which strongly suggests that at least some chromosomes in these genomes are organised non-randomly at metaphase. Of particular interest was the highly significant X-5 association in the MMX genome since in a previous study X-5 rearrangements were found to occur repeatedly among different backcross progeny involving Moreton x Torresian F1 hybrids. Additionally a comparison of the organisation of chromosomes in the MAX and MMX genomes, which differ primarily by the type of X chromosome, revealed that in a number of cases pairs of chromosomes are arranged very differently with respect to each other. The distribution of chromosomes on the hollow spindle was also analysed to investigate whether a specific spatial ordering of chromosomes exists within these Moreton genomes based on the association of pairs of short arms and pairs of long arms of most similar length (the Bennett model). The twelve chromosomes in both genomes were uniquely ordered in a single chain. However, because of computing limitations, only the ordered arrangement of chromosomes 1–10 was investigated. An analysis of 48 cells in the MMX and 38 cells in the MAX genomes showed that the predicted order in the ten chromosome sub-set in each genome did not rank in the top 20% of the 181,440 possible orders. This suggests that, although there is a good evidence that some non-homologous chromosomes may be associated non-randomly at metaphase in these genomes, they do not appear to show a specific, ordered arrangement as predicted by the Bennett model. The significance of the observed non-random organisation of chromosomes in the MMX and MAX genomes is discussed in relation to the generation of novel chromosome rearrangements in Moreton x Torresian F1 hybrids and the evolution of the Moreton and Torresian genomes.