Aberrant spermatogenesis and the peculiar mechanism of sex determination in Symphypleonan Collembola (Insecta)

Light 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 = 10; 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:X1X20. 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.     

Brain organization in Collembola (springtails)

Arthropoda is comprised of four major taxa: Hexapoda, Crustacea, Myriapoda and Chelicerata. Although this classification is widely accepted, there is still some debate about the internal relationships of these groups. In particular, the phylogenetic position of Collembola remains enigmatic. Some molecular studies place Collembola into a close relationship to Protura and Diplura within the monophyletic Hexapoda, but this placement is not universally accepted, as Collembola is also regarded as either the sister group to Branchiopoda (a crustacean taxon) or to Pancrustacea (crustaceans + hexapods). To contribute to the current debate on the phylogenetic position of Collembola, we examined the brains in three collembolan species: Folsomia candida, Protaphorura armata and Tetrodontophora bielanensis, using antennal backfills, series of semi-thin sections, and immunostaining technique with several antisera, in conjunction with confocal laser scanning microscopy and three-dimensional reconstructions. We identified several neuroanatomical structures in the collembolan brain, including a fan-shaped central body showing a columnar organization, a protocerebral bridge, one pair of antennal lobes with 20-30 spheroidal glomeruli each, and a structure, which we interpret as a simply organized mushroom body. The results of our neuroanatomical study are consistent with the phylogenetic position of Collembola within the Hexapoda and do not contradict the hypothesis of a close relationship of Collembola, Protura and Diplura.     

Global diversity of springtails (Collembola; Hexapoda) in freshwater

Water-dependency appeared independently in several clades of the class Collembola, which is basically of terrestrial origin according to recent phylogenetic analyses. Though moderately diversified (less than 8,000 species), Collembola are among the most numerous terrestrial arthropods in wetland communities, with a small number of species living on the surface of water. Many species are dependent on water-saturated atmosphere of caves, and on snow or ice in high mountains. A total of 525 water-dependent species have been recognized, of which 103 are linked to free freshwaters and 109 to anchialine or marine waters. Many interstitial species are also dependent to an unknown extent on water saturation in the deep layers of the soil. The numbers provided here are underestimates, as Collembola are extremely poorly known outside the Holarctis, and the ecology of described species usually not documented. However, a general biogeographical pattern is emerging from available data. The most remarkable feature is that about 15% of the fauna is water-dependent in the holarctic region, compared to 4% in the tropics and southern hemisphere. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Guest editors: E. V. Balian, C. Lévêque, H. Segers & K. Martens Freshwater Animal Diversity Assessment     

Das Doppelauge vonEntomobrya muscorum Nicolet (Insecta,Collembola)

Zusammenfassung Der bodenlebende CollembolaEntomobrya muscorum hat an jeder Kopfseite ein loses Aggregat von acht Ommatidien, von denen zwei einen deutlich kleineren Linsendurchmesser aufweisen. Die sechs großen Ommatidien werden als Haupt-, die zwei kleinen als Nebenaugen bezeichnet. Beide Ommatidientypen haben verschieden gebaute Rhabdome, so daß der Komplex ein Doppelauge darstellt. Es wird der Bau der Hauptaugen geschildert. Ihr sensibler Teil setzt sich aus acht Retinulazellen zusammen, die ein offenes Rhabdom in zwei Lagen bilden. Eine zentrale sechste Zelle in der distalen Schicht weicht in ihrer Feinstruktur deutlich von den übrigen ab. Die Rhabdome beider Ommatidientypen werden miteinander verglichen, und ihre funktionelle Bedeutung vor allem in Hinblick auf eine mögliche Arbeitsteilung für eine Polarisationswahrnehmung wird erörtert. Es wird kurz die phylogenetische Bedeutung dieser Augen für die Insekten und die Collembolen selbst angeschnitten.

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The double eye ofEntomobrya muscorum nicolet (Insecta, Collembola)

Summary The ground living CollembolaEntomobrya muscorum has on each side of the head eight circular ommatidia in an arrangement characteristic of the species. Of these two have a distinctly smaller lens. The six big ommatidia are called primary eyes, the two small ones accessory eyes. The two types of ommatidia have different rhabdom structures, so justifying the term double eye. The structure of the primary eyes is described. The photosensitive part is constructed of eight retinula cells which form an open rhabdom in two layers. The central sixth cell in the distal layer is quite different from the others. The rhabdoms of both types are compared. The possible function of the accessory eyes in perception of the e-vector is discussed. The phylogenetic significance of these eyes for the Insecta and the Collembola is considered.

     

Checklist of the springtails (Hexapoda: Collembola) of Turkey

A checklist of the species of springtails (Hexapoda: Collembola) hitherto recorded from Turkey is presented. This list contains 53 species belonging to 39 genera in 13 families. The diversity of Collembola in Turkey is poorly known and it is clear that numerous species wait to be discovered.     

Aberrant spermatogenesis and sex determination in Bourletiellidae (Hexapoda, Collembola), and their evolutionary significance

Light and electron microscopic evidence is provided to describe a new example of a postzygotic sex-determination system in two collembolan species, Bourletiella arvalis and B. hortensis. In B. arvalis, where chromosome number could be assessed, both sexes are homogametic (n=6) and all zygotes have an identical chromosome composition (2n=12). However, male embryos develop after the loss of two sex chromosomes, making the male genotype 2n=10 (4AAX₁0X₂0). On the other hand, female embryos develop if the zygote retains all chromosomes and the female genetic system is, therefore, 4AAX₁X₁X₂X₂ (2n=12). As an apparent consequence of the lack of two chromosomes in the male germ cells, spermatogenesis is aberrant. At the first meiotic division, in fact, the two resulting secondary spermatocytes receive a different number of chromosomes: six and four. The cells which receive six chromosomes (one haploid set of four autosomes and two sex chromosomes) proceed through the meiotic process and the two spermatids generated produce two spermatozoa by a normal spermiogenesis. The cells receiving only four chromosomes do not undergo the second meiotic division and soon degenerate. The degenerating cells can be considered a morphological marker for this process, as they are easily recognizable at the electron microscope from the functional secondary spermatocytes by the appearance of the nucleus (totally condensed), the reduction of the cytoplasm (limited to a thin layer surrounding the nucleus), and the lack of most cytoplasmic organelles (with the exception of a couple of centrioles). Electron microscopic evidence has been collected for both species, allowing to extend the same process to B. hortensis, even if chromosomes could not be counted in this species. Therefore, as a result of the spermatocyte elimination, the efficiency of spermatogenesis is reduced to 50%. This process is identical to that observed in other collembolan species of the suborder Symphypleona, and it is suggested that it represents a synapomorphic feature uniting the families Dicyrtomidae, Sminthuridae and Bourletiellidae (Sminthuriformia). It is also suggested that the process is related with the finding of a distorted sex ratio in natural populations and, possibly, with the evolution of parthenogenesis. This hypothesis is supported by the fact that chromosome pairing and genetic recombination occurs only during female meiosis, while chromosomes do not pair during male meiosis. Accepted: 27 December 2000     

Chromosome elimination in germ line-soma differentiation of Acricotopus lucidus (Diptera, Chironomidae).

During germ line-soma differentiation in early syncytial embryonic development of the chironomid Acricotopus lucidus, a complement of supernumerary chromosomes, the so-called germ line limited chromosomes (Ks), is excluded from the future somatic nuclei in the course of elimination mitoses. The Ks lag behind in the equatorial plane, while the somatic chromosomes (Ss) segregate equally. After elimination mitoses, the Ks are only present in the pole cells, the primary germ cells. In the divisions before their elimination, the Ks frequently showed delayed separation of sister chromatids with high-frequency formation of anaphasic bridges and lagging in pole movement as detected in 4',6-diamidino-2-phenylindole (DAPI)-stained squash preparations of early embryos. To determine if all of the Ks are eliminated in one step during a single mitosis, a fluorescence in situ hybridization (FISH) analysis of early embryonic divisions was performed using probes of germ line specific repetitive DNA sequences, which specifically label the Ks in their centromeric regions. In most cases, all of the Ks are lost in one mitosis; however, occasionally one or several of the Ks can escape their elimination by segregating and moving poleward together with the Ss. The escaping Ks will then be eliminated in one of the following mitoses. This clearly indicates that the specific conditions to eliminate Ks are not restricted to only one division. Possible mechanisms of elimination of Ks are discussed.     

Spatial patterns in the distribution of European springtails (Hexapoda: Collembola)

Using a large database on the spatial distribution of European springtails (Collembola) we investigated how range sizes and range distribution across European countries and major islands vary. Irrespective of ecological guild, islands tended to contain more endemic species than mainland countries. Nestedness and species co‐occurrence analysis based on country species lists revealed latitudinal and longitudinal gradients of species occurrences across Europe. Species range sizes were much more coherent and had fewer isolated occurrences than expected from a null model based on random colonization. We did not detect clear postglacial colonization trajectories that shaped the faunal composition across Europe. Our results are consistent with a multiregional postglacial colonization. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 498–506.     

Chromosome number and sex determination coevolve in turtles

Although much progress has been achieved in understanding the genetic basis of adaptation, the drivers of genome evolution remain obscure. For instance, extensive variation among reptilian genomes continues largely unexplained, yet reptiles hold critical clues about vertebrate evolution. Turtles posses diverse chromosome numbers (2N = 28-66) derived from extensive genomic rearrangements, plus varied sex-determining mechanisms (genotypic and temperature-dependent). Here, we show that rates of evolution in turtle chromosome number are ~20-fold higher along phylogenetic branches where transitions between sex-determining mechanisms also occur, revealing a strong coevolution of these traits and making drift a less likely driver. Directional tests indicate that both traits evolved effectively in synchrony. These events occurred near global extremes in temperature shifts over the last 200 million years, although the role of climate change remains unknown at this point. Two alternative testable explanations for these patterns are proposed. First, selection for sex determination turnover may co-opt mechanisms (e.g., chromatin remodeling) favoring genomic rearrangements. Alternatively, chromosomal rearrangements underlying diploid number evolution may alter gene regulation enabling transitions in sex-determining mechanisms. Our data indicate that the evolution of sex determination is intimately linked to profound genomic changes underlying diploid number evolution, the ecological context of which remains intriguing.     

中国水生跳虫新种及新记录种记述 (弹尾目)

自Uchida(1948)从山西东冶县发现中国第1种水生Podura aquatica Linnaeus, 1785以来,中国弹尾目水生种类再未见报道。该文记述了采自上海植物园水塘静水表面的弹尾目5新种,水生原Podura aquatica, 尹氏球角Hypogastrura yinae sp. Nov., 沼生陷等Isotomurus palustris, 水生握角圆Sminthurides aquaticus 及刺拟勃氏圆Pseudobourletiella spinata。 其中水生握角圆和刺拟勃氏圆为中国新记录种,尹氏球角为新种。新种的模式标本及观察的其它种的标本存放在中国科学院上海昆虫研究所标本馆。     

Environmental correlates of species richness of European springtails (Hexapoda: Collembola)

Our knowledge about environmental correlates of the spatial distribution of animal species stems mostly from the study of well known vertebrate and a few invertebrate taxa. The poor spatial resolution of faunistic data and undersampling prohibit detailed spatial modeling for the vast majority of arthropods. However, many such models are necessary for a comparative approach to the impact of environmental factors on the spatial distribution of species of different taxa. Here we use recent compilations of species richness of 35 European countries and larger islands and linear spatial autocorrelation modeling to infer the influence of area and environmental variables on the number of springtail (Collembola) species in Europe. We show that area, winter length and annual temperature difference are major predictors of species richness. We also detected a significant negative longitudinal gradient in the number of springtail species towards Eastern Europe that might be caused by postglacial colonization. In turn, environmental heterogeneity and vascular plant species richness did not significantly contribute to model performance. Contrary to theoretical expectations, climate and longitude corrected species–area relationships of Collembola did not significantly differ between islands and mainlands.     

Ultrastructure of the male accessory glands ofAllacma fusca(Insecta, Collembola)

The accessory glands ofAllacma 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.     

Polymorphism of polytene chromosomes in Bilobella aurantiaca (Insecta: Collembola)

Polytene chromosomes in the salivary glands of Bilobella aurantiaca are analysed in a population from Sierra de Gredos (Central Spain). They display a high degree of heterozygosity. Only a short part of the total length of the seven pairs of homologous chromosomes, examined in a single favourable specimen, was homozygous. Concerning the short arm of chromosomes III and IV, the respective proportion of heterozygosity for the whole population was 38.6% and 92.9%. Some hypotheses are formulated to account for these results: differential funtioning of homologous chromosomes, generalisation of minute duplications and deficiencies, complex inversions (which cannot be specified owing to the asynapsis of homologous chromosomes), and possibly local large deletions.     

Environmental correlates of body size distributions of European springtails (Hexapoda: Collembola)

Aim Species–body size distributions (SBDs) are plots of species richness across body size classes. They have been linked to energetic constraints, speciation–extinction dynamics and to evolutionary trends. However, little is known about the spatial variation of size distributions. Here we study SBDs of European springtails (Collembola) at a continental scale and test whether minimum, average and maximum body size and the shapes of size distributions change across latitudinal and longitudinal gradients and whether SBDs of islands and mainlands differ. We also test whether the island rule and the positive body size–range size relationship of vertebrates also holds for Collembola. Location Europe. Methods We use a unique data set on the spatial distributions of 2102 species of European springtails across 52 countries and larger islands together with associated data on body size, area, climate variables, longitude and latitude. Differences in the central moments of SBDs are inferred from simultaneous spatial autoregression models. Results The SBD of the European Collembola and its largest suborder Entomobryomorpha is unimodal and symmetrical. Average, minimum and maximum body weight and the skewness of the mainland/island SBDs peaked at intermediate latitudes. We could not find simple latitudinal gradients in minimum and maximum body weight. Average and maximum body size increased with country/island area in accordance with the island rule in vertebrates, while minimum body size did not significantly differ between islands and mainlands. Finally, we found a weak but statistically significant positive correlation of range size and body size. Main conclusions We provide evidence for differences in body size distributions between islands and mainlands that are in part in line with the island rule in invertebrates. We also find evidence for an interspecific body size–range size relationship similar to that of vertebrates although the vertebrate pattern is much stronger than the springtail pattern. Our results on latitudinal gradients of maximum and average body size imply the need to account for species richness and area effects in the study of latitudinal gradients in body size. We recommend implementing sample size and area effects in the study of body size distributions on islands and mainlands.     

Dicyrtotnina and Ptenothrix (Insecta:Collembola) of the Solomon Islands

This paper deals primarily with the collembolan species of the subfamily Dicyrtominae collected during the Royal Society Expedition to the Solomon Islands of 1965 by P. N. Lawrence, supplemented by material from these islands collected by Dr and Mrs P. J. M. Greenslade, all of which is in the British Museum (Natural History). Additional supporting material described here has been derived from the Noona Dan Expedition of the Zoological Museum, Copenhagen and from the Bernice P. Bishop Museum, Hawaii. I should like to express my thanks to the individuals and Museum sconcerned.     

.Chromosome sex determination and Y-autosome fusion in Blennius tentacularis Brunnich, 1765 (Pisces, Blennidae)

Diploid modal numbers of 2n =48 for females, and 2 n =48 and 2 n =47 for males of Blennius tentacularis caught in the Gulf of Palermo (Sicily) are established. Chromosome sex-determination is proposed for this species in which a Y-autosome fusion has been found.     

A New Species of Halophilous Collembola (Insecta) from Korea

Three species of interstitial Collembola in Entomobryidae were identified from South Korean coast. One of them, Sinella (Coecobrya) maritima, is new to science. So far 21 halophilous species in 10 families of Collembola, including the new species in this paper, were reported from the Korean peninsula.