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Chromosome elimination and sex determination in springtails (Insecta, Collembola)
No full textA post-zygotic mechanism of sex determination is described in the two symphypleonans Dicyrtomina ornata (Nicolet) and Ptenothrix italica Dallai. The process consists of the loss of two sex chromosomes from the male embryo. At the end of the first meiotic division of spermatogenesis, a second chromosome elimination occurs, allowing half the secondary spermatocytes, later transformed into spermatids, to receive a complete haploid set of chromosomes. The secondary spermatocytes, which receive an incomplete set of chromosomes, degenerate. Males of the two collembolan species, therefore, produce a reduced number (50%) of spermatozoa. Females of D. ornata have 2n = 12 and males 2n = 10 chromosomes; females of P. italica have 2n = 14 and males 2n = 12 chromosomes. In both species, oogenesis proceeds normally and chromosomes pair and form chiasmata in meiotic prophase. The adaptive significance of this post-zygotic mechanism of sex determination is discussed. The mechanism seems to be a characteristic feature of the suborder Symphypleona. The neanurid Arthropleona Anurida maritima (Guérin), which was studied for comparative analysis, has 2n = 8 chromosomes and normal spermatogenesis producing haploid nuclei with four chromosomes
Allozyme variation in reference and metal-exposed natural populations of Orchesella cincta (L.) (Insecta, Collembola)
No full textEnvironmental pollution may affect genetic variation in populations inhabiting polluted sites. In this study, allozyme variation was studied in natural populations of the soil-dwelling insect Orchesella cincta. The populations originated from eight sites with different histories of metal contamination and natural enrichment in The Netherlands, Belgium and F.R.G. Slight but significant divergence was observed for four polymorphic loci; 18 loci were monomorphic at all sites. The highest mean values for heterozygosity (H), degree of polymorphism (P) and mean number of alleles per locus (A) were 0.090, 0.182 and 1.36, respectively. Geographic distance was not correlated to genetic divergence. Comparison of allozyme data from investigated populations and from a reference population in Italy showed a remarkable genetic homogeneity in this species, the highest value for Nei's genetic distance (D) being 0.035. Among NW European populations, a correlation between metal tolerance characteristics and allozyme frequencies was observed for glutamate-oxaloacetate transaminase (EC. 2.6.1.1)
New data on the aberrant spermatogenesis of Collembola
No full textAberrant spermatogenesis in the two species Stenognathellus denisi (Katiannidae) and Sminthurides aquaticus (Sminthurididae) is described. An ultrastructural study of spermatogenesis in these species has allowed us to reveal the presence of two types of secondary spermatocytes; one with a normal appearance, the other one is characterized by a small size, a reduced cytoplasm (showing only two centrioles), and a nucleus with very condensed chromatin. These latter aberrant cells are not able to perform the second meiotic division and will degenerate. This finding allows us to extend the identification of an aberrant spermatogenesis and the consequent postzygotic sex determination to two more families of Symphypleona
Aberrant spermatogenesis and the peculiar mechanism of sex determination in Symphypleonan Collembola (Insecta)
No full textLight and electron microscopy evidence have been obtained to describe the peculiar spermatogenesis in the collembolan species Sminthurus viridis and Allacma fusca (Sminthuridae), In these two species, the two sexes differ for the lack of two chromosomes (the sex chromosomes) in males (males, 2n = in; females, 2n = 12), While oogenesis seems to proceed normally, spermatogenesis is peculiar because the two daughter cells of the first meiotic division have different chromosome numbers (six and four). The cell receiving four chromosomes degenerates, while the cell receiving six chromosomes completes meiosis and produces identical spermatozoa (n = 6), At fertilization, pronuclei with six chromosomes fuse together to form zygotes with 2n = 12, Male embryos must lose two sex chromosomes during the first zygotic mitosis, as all male cells have 2n = 10 chromosomes. The sex chromosome system of these species can be identified as X1X1X2X2:X(1)X(1)0. Electron microscopy observations show that the same peculiar spermatogenesis occurs also in two others species of the same family, Caprainea marginata and Lipothrix lubbocki, The peculiar sex determination system described is similar but not identical to what is observed in other insect orders, and it may represent an evolutionary step toward parthenogenesis, It is suggested that this peculiar spermatogenesis is common to all Symphypleona
Comparative analysis of two edaphic zoocenoses (Acari Oribatida; Hexapoda Collembola) in the area of Orio al Serio Airport (Bergamo, northern Italy)
No full textSpecies richness, abundance and diversity of oribatid mites and springtails in soil and litter were investigated at four sites in the area of Orio al Serio Airport (Bergamo, Italy). A total of 60 oribatid mite and 35 springtail species were collected from the sites. The general low diversity and evenness along with high dominance may be related to the sensitivity of these edaphic zoocoenoses to soil impoverishment which has been occurring in this part of the Italian pre-Alps for many years. Quantitative and qualitative differences were found among the investigated areas. Results show that less frequently disturbed soils are capable of restoring oribatid and springtail populations, leading to greater equilibrium. Results show that less frequently disturbed soils allow the restoration of more equilibrated oribatid and springtail populations. Species were divided into 8 autoecological and 11 chorological categories. Correspondence Analysis arranged these categories among the sites, confirming the above-mentioned trend.Species richness, abundance and diversity of oribatid mites and springtails in soil litter were investigated at four sites in the area of Orio al Serio Airport (Bergamo, Italy). A total of 60 oribatid mite and 35 springtail species were collected from the sites. The general low diversity and eveness along with high dominance may be related to the sensitivity of these edaphic zoocenoses to soil impoverishment which has been occurring in this part of the italian pre-Alps for many years.Quantitative and qualitative differences were found among the investigated areas. Results show that less frequently disturbed soils are capable of restoring oribatid and springtail population, leading to greater equilibrium. Results show that less frequently disturbed soils allow the restoration of more equilibrated oribatid and springtail populations. Species were divided into 8 autoecological and 11 chorological categories, Correspondance Analysis arranged these categories among the sites, confirming the above-mentioned trend
Genetic diversity and taxonomy in soil dwelling insects: the genus Orchesella
No full textGenetic diversity and inter- and intraspecific differentiation are evaluated in 10 populations of four different species of the soil-dwelling genus Orchesella (Collembola, Entomobryidae). The degree of genetic variability is consistent with that observed in other insects and soil organisms. Values of genetic distance are very high and suggest that the species differentiated a long time ago. Two different procedures are used to estimate the rate of gene flow among six populations of O. villosa, but they give contradictory results: this species seems to be subdivided into two groups of populations (inland and coastal), which might represent two subspecies
High genetic divergence among populations of Tetrodontophora bielanensis (Insecta, Collembola) in Europe
No full textTen populations of Tetrodontophora bielanensis from Poland to northeastern Italy are investigated using isozyme variability, demonstrating that a relevant genetic divergence is present among them
High levels of genetic variability and population differentiation in Gressittacantha terranova (Collembola, Hexapoda) from Victoria Land, Antarctica
No full textAllozyme electrophoresis was used to asses genetic variability and differentiation in 22 populations of Gressittacantha terranova Wise (Hexapoda, Collembola) from a coastal area of Victoria Land between the Mariner Glacier and the Nansen Ice Sheet. Allelic frequencies were determined at five enzyme loci: Phi, Pgm, Hk, Mpi and Mdh. Levels of variability, estimated as rates of heterozygosity, were higher than those calculated for the same loci in taxonomically related and non-related species of non-Antarctic Collembola. Thus, in spite of the ecological simplicity of Antarctic terrestrial ecosystems, G. terranova is characterized by high levels of genetic variability, and the 22 populations could be divided into three geographic groups, separated by the Aviator and Campbell glaciers. Genetic differentiation reflects the geographic arrangment of the populations, suggesting that the glaciers are effective barriers to gene flow, and that the patchy distribution of collembolan species in Antarctica has the potential to induce, in the long term, microspeciation processes. Interestingly, detectable genetic differentiation was observed between six populations collected at Edmonson Point, even though these are very close to each other, indicating the impact of geographic isolation even within short distances. The only exception to the congruence between genetic and geographic structuring was provided by the population of Apostrophe Island, for which a recent introduction with individuals coming from southern populations is suggested
Ultrastructure of the male accessory glands of Allacma fusca (Insecta, Collembola)
No full textThe accessory glands of Allacma fusca(L.) (Insecta, Collembola, Sminthuridae) consist of a series of
secretory units that are arranged in parallel and open into the ejaculatory duct. Each unit is composed of microvillate
cells stacked around a common cavity. Basal cells are involved in ion-control of fluids from the hemocoel to
the cavity. The intermediate and apical cells, which have a laminar appearance and contain many microtubules,
are involved in the structural integrity of the unit. Supporting cells ensheath the most apical cells. Large openings
in the cuticle allow the gland secretion to flow into the ejaculatory duct lumen. These openings are protected by a
porous cuticle different from that lining the epithelium of the ejaculatory duct. Conspicuous muscle fibers run
along the lateroventral side of the ejaculatory duct beneath the insertion of the accessory glands. The fine structure
of the accessory glands indicates that they are type I ectodermic glands as defined by Noirot & Quennedey
(1974). Their function could be to control the fluidity of the material for spermatophore formation and to ensure the
proper physiological conditions for spermatozoa stored in the ejaculatory duct lumen
The use of genetic markers for the diagnosis of sibling species in the genus Isotomurus (Insecta, Collembola)
No full textTwo independent genetic markers (allozymes and mitochondrial DNA sequences) were used to demonstrate that Isotomurus palustris palustris and Isotomurus palustris maculatus (Collem- bola, Isotomidae), interpreted either as varieties or subspecies by the various authors, are, actually, well differentiated species. Specimens were collected sympatrically in five different sampling sites in Italy and southern France, and, in all of them, the two forms were fixed for alternative alleles at some enzyme loci. The absence of hybrids demonstrates that the two species, discriminated morphologically by body pigmentation, do not cross. The rate of sequence divergen- ce of the mitochondrial COII gene confirms a significant degree of differentiatio
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