Flow-cytometric analyses of intraspecific genome size variations in Bacillus atticus (Insecta, Phasmatodea)

Abstract

The stick insect Bacillus atticus comprises several populations with different chromosome numbers that are distributed over a large range of the Mediterranean basin. Here we have analyzed the DNA content of nine diploid and three triploid populations by flow-cytometry. The mean genome size of the diploids showed a significant decrease from east to west, ranging from 5.29 ± 0.12 pg for the population from Crete (east) to 4.28 ± 0.10 pg for the population from Sardinia (far west). This longitudinal trend of a decrease in genome size from east to west was also found for the triploid populations (from 6.80 pg for the population in Turkey to 6.08 ± 0.01 pg for the population on the Isle of Rhodes). Differences in DNA content between populations belonging to the same species have been described in animals, but the evolutionary implications of these differences are as yet unclear. What emerges from the present study is a correlation between genome-size variations and geographic distribution. The adaptive nature of genome-size variations in response to environmental changes is discussed, and the class of DNA involved hypothesized.Résumé : Le phasme Bacillus atticus comprend plusieurs populations qui montrent un nombre chromosomique variable et qui sont distribuées sur la plupart du bassin méditerranéen. Dans la présente étude, les auteurs ont analysé le contenu en ADN de neuf populations diploïdes et de trois populations triploïdes par cytométrie en flux. La taille moyenne du génome chez les diploïdes diminuait d'est en ouest, allant de 5,29 ± 0,12 pg chez la population de Crète (est) à 4,8 ± 0,10 pg chez la plus occidentale (Sardaigne). Cette tendance longitudinale de diminution de la taille du génome d'est en ouest a aussi été observée chez les populations triploïdes (de 6,80 pg en Turquie jusque 6,08 ± 0,01 pg sur l'Île de Rhodes). Des différences intraspécifiques quant au contenu en ADN ont été décrites chez les animaux mais leurs implications évolutives demeurent nébuleuses. Il ressort de cette étude une corrélation entre les variations de la taille du génome et la distribution géographique. La plasticité adaptative que confèrent des variations du contenu en ADN face à des changements environnementaux ainsi que la classe d'ADN qui est vraisemblablement impliquée sont discutées.Mots clés : valeur C, cytométrie en flux, tendances quant à la taille du génome, variation intraspécifique du contenu en ADN, classes d'ADN.

Genome size of the northern walkingstick, Diapheromera femorata (Phasmida: Heteronemiidae)

“…Based on an average of all values calculated from both the haemocyte (n = 12) and sperm (n = 5) samples, the genome size of D. femorata is estimated at 1C = 2.55 ± 0.06 pg (mean ± standard error). This is within the range of 1C values, 1.95-2.94 pg, for representatives of the genus Bacillus obtained by means of flow-cytometric measurement of midgut cells versus a mouse thymocyte standard (Marescalchi et al 1990(Marescalchi et al , 1998. The possibility that phasmid genomes are consistently large across different families is supported by this observation, though much work remains to be done before any generalizations can be made regarding a "typical" genome size for the order as a whole.…”

“…European populations of Bacillus atticus consist of both diploid and triploid parthenogenetic strains, and among the former there is significant latitudinal variation (~20%) in nuclear DNA content, which decreases gradually from east to west (Marescalchi et al 1998). By contrast, D. femorata is bisexual, and the consistent DNA contents of male and female somatic cells and sperm indicate a likely diploid state in this species.…”

“…Whether large genomes such as this are typical of phasmids remains to be seen. Thus far, the genomes of representatives from only one other genus of stick insects (Bacillus, family Phasmatidae) have been measured, and range in size from about 1C = 2 pg to 1C = 3 pg (four species and seven subspecies in total; Marescalchi et al 1990Marescalchi et al , 1998. Unfortunately, this lack of data is symptomatic of insects at large, of which a mere 300 species have been analyzed in terms of genome size, the majority of them flies, beetles, grasshoppers, and aphids (Gregory 2001).…”

Chromosome rearrangements are not accompanied by expected genome size change in the tree weta Hemideina thoracica (Orthoptera, Anostostomatidae)

“…Increase in DNA content without major chromosomal change is common and usually attributed to increased quantities of noncoding repetitive DNA (Gregory & Hebert 1999). Intraspecific continuous increases in DNA content have been associated with climatic gradients and age-of-occupancy gradients (Jockusch 1997, Marescalchi et al 1998. The trend seen in this dataset of relatively more DNA with fewer chromosomes (Fig.…”

Small, but surprisingly repetitive genomes: Transposon expansion and not polyploidy has driven a doubling in genome size in a metazoan species complex

“…The authors identified two polyploidy events in the history of this family, and found that transposable element content was influenced by these ploidy changes across the species studied (13). Examples of genome size variation on closer evolutionary scales have been identified in a few animals species (14)(15)(16), but detailed genomic examination of these cases is rare. Some species of snapping shrimp have been found to exhibit genome size 4 variation that does not appear to be caused by polyploidy, but has not been further characterised (17).…”