1,721,271 research outputs found
Genome exclusion and gametic dapi—dna content in the hybridogenetic Bacillus rossius—grandii benazzii complex (insecta phasmatodea)
No full textAmong Sicilian stick insects, two hybridogenetic complexes have been discovered: Bacillus rossius–grandii benazzii and B. rossius–grandii grandii, which also produce androgenetic offspring. The egg maturation of the former is analyzed here through DAPI fluorom etry, which, besides the assessment of the meiotic stages, also allows their DNA measurements and the analysis of sperm‐head evolution into male pronuclei in these polyspermic eggs. Hybridogenetic eggs undergo an extrasynthesis of chromosomes, because two groups of n autobivalents (4C each) are segregated at metaphase 1st; the two groups must correspond to the pure parental species haplosets. Then the grandii chromosomes degenerate (1st polar body), while the rossius chromosomes divide further to produce two groups of n autodiads (2C each); one of them degenerates (2nd polar body), and the other is ready to perform syngamy (female pronucleus). Meanwhile, several B. grandii sperm evolve into male pronuclei by doubling their DNA (from 1C to 2C content) and assuming an interphase nucleus appearance. If regular mixis occurs, the F1 hybrid constitution is restored but, if it fails, a fusion between two sperms may occur, originating fully paternal descendants (natural androgenesis). The genome exclusion mechanism of stick‐insect hybridogens appears to be more primitive than those observed in the already known hybridogenetic complexes of Poeciliopsis and Rana esculenta. Unfertilized eggs of hybridogens are capable of self activation, but the cytology of the related clonally reproducing B. whitei indicates that its parthenogenetic mechanism stems from the hybridization event (hybrid theory) rather than from tychoparthenogenetic potentialities (spontaneous theory). © 1992 Wiley‐Liss, Inc. Copyright © 1992 Wiley‐Liss, Inc
Reproduction and genetic variation in clam shrimps (Crustacea, Branchiopoda, Conchostraca)
No full textSamples of the three clam shrimp species. Eoleptestheria ticinensis and Leptestheria dahalacensis (Leptestheriidae) and Limnadia lenticularis were collected in north Italian rice paddies in Emilia and Lombardy. Sex-ratio analysis revealed that in the two species of Leptestheriidae, males are more frequent than females, whereas L. lenticularis has an all-female constitution. Allozyme analysis showed a low degree of genetic variation, the values of polymorphism parameters being very similar to those found for other Branchiopoda. In the Leptestheriidae it was generally found that genotypic frequencies conform to Hardy-Weinberg expectations, demonstrating a high outcrossing rate. No clear-cut relationship between the excess of males and cytological and genetic observations could be established, since neither sex chromosomes nor sex-linked genes were found. Nevertheless, male predominance could overcome the morphodynamic constraints that clam shrimps face during fertilization. The all-female constitution of the two populations of L. lenticularis and the hatching of unisexual progeny from virgin females suggest that they reproduce by obligate parthenogenesis; the almost complete lack of heterozygous genotypes at all the analyzed loci allows us to discard the previous hypothesis of automixis. Further analyses should be undertaken to fully clarify the sex-determination mechanisms operating in these clam shrimps, as was previously done in Eulimnadia texana
Molecular systematics of the Atlanto-Mediterranean Solea species
No full textWithin the genus Solea, four sister lineages have been evolved that actually correspond to the species S. vulgaris, S. senegalensis, S. kleini and S. lascaris. The existence of S. aegyptiaca and S. impar is not supported and these species can be synonymized under S. vulgaris and S. lascaris, respectively. This pattern of taxonomy agrees fully with that proposed by Ben-Tuvia (1990; Journal of Fish Biology 36, 947-960) based on morphological features. In addition, mtDNA data contradict the widely accepted relatedness between S. kleini and S. lascaris based on the shared enlarged forenostril on the blind side. A sample of soles clearly related to S. senegalensis was caught in the Gulf of Taranto (Ionian Sea), representing the most western finding of this species in the Mediterranean Sea. (C) 2000 The Fisheries Society of the British Isles
Androgenetics and triploids from an interacting parthenogenetic hybrid and its ancestors in stick insects
No full textPopulalions of unisexual organisms are often assumed to be genetically invariant (clones) and destined to a short existence on an evolutionary timescale. Unisexual organisms are most often obligate parthenogens and, by definition, ought to be completely isolated reproductively from related bisexual organisms. The assumption of complete reproductive isolation between amphinmictic ancestors and thelytokous hybrids is common to most hypotheses on the evolution of sex and its adaptive significance. Stick insects of the genus Bacillus however provide evidence for reproductive interactions between allodiploid parthenogens and their ancestors, because pure species progeny (androgenetics) and triploid descendants are produced. These findings demonstrate that, through androgenesis, offspring of parthenogenetic hybrid females can contribute specimens of both sexes to the fathering species when fertilized by syntopic ancestral males and the parthenogenetic egg of strictly clonal females, when fertilized, allows a third genome to be added to the allodiploid chromosome set. These triploid genomes promote further genetic diversification and evolution of the unisexual populations through the formation of new clones by recombination during the changed maturation mode of allotriploid eggs. All this argues for much more complex breeding systems and evolutionary pathways than are usually assumed for hybrid unisexual organisms
Chromosomal evidence of hemiclonal and all-paternal offspring production in Bacillus rossius-grandii benazzii (Insecta Phasmatodea)
No full textThe stick insects Bacillus rossius-grandii benazzii and B. rossius-grandii grandii naturally reproduce by hybridogenesis and androgenesis. The hybrid karyotype of the former (2 n=35, XX female; 34, X0 male) clearly sums up a B. rossius haploset (r) with n=18 and a B. grandii benazzii one (gb) with n=17. The two sets keep the parental features for C-heterochromatin amount (much larger in the gb complement) and satellites/NORs (nuclear organizer regions) (more numerous and variably located in the r set); hybridogenetically produced males always show severely impaired gametogenesis and are therefore sterile, whereas hybridogenetic females are fertile. Reproductive, karyological and cytogenetical properties of the hybridogenetic system have been exploited to obtain the chromosomal evidence of whole haploset exchanges. In progenies obtained by crossing B. rossiusgrandii benazzii females to B. rossius males with either standard or repatterned (with Robertsonian fusions) karyotypes, there has always been complete agreement between electrophoretically genotyped and karyologically analyzed hybridogenetic offspring: the unassorted maternal r haploset (rm) is transmitted and the gbm haploset replaced by that of the fathering male (rp), thus evidencing the hemiclonal reproduction and the new rm-rp chromosomal constitution. New karyotype traits of the offspring relate to chromosome number (2 n=36, female; 35, male), C-heterochromatin pattern (the heterochromatin-rich gb haploset completely disappears) and satellite/NOR features (corresponding to rm plus rp locations). The same crosses also produce genetically and chromosomally all-paternal descendants (androgenetics), of both sexes and fully fertile, with an rprp structure. These androgenetic progeny show segregation for alleles and chromosomes at which fathering males are heterozygous: it was therefore possible to demonstrate that androgenetics can derive from syngamy of two sperm nuclei, of the several present in the polyspermic hybridogenetic egg. The production of androgenetics from field fertilized females of B. rossius-grandii benazzii, B. rossius-grandii grandii and parthenogenetic Bacillus whitei (=B. rossius/grandii grandii) suggests the occurrence of unsuspected relationships between hybrids and their parental species, so that the hybrids cannot be simply considered as 'sexual parasites'. Furthermore, there is a suggestion of evolution of parthenogenetic clonal species from selection of initially hybridogenetic strains. The ability to produce uniparental progeny naturally from the spermatic genome may open a new field of investigation on genomic imprinting. © 1993 Springer-Verlag
Satellite DNA variation in parental and derived unisexual hybrids of Bacillus stick insects (Phasmatodea)
No full textThe Bag320 sequence family of satellite DNA (satDNA) has been found in some stick insect taxa- the bisexual Bacillus grandii, the related parthenogen B. atticus and their hybrids with B. rossius. However, under the same experimental conditions, the Bag320 sequences were not found in B. rossius. Bag320 sequences of the clonal hybrid B. whitei (= B. rossius/grandii grandii) intermingled with those of B. grandii in all plotted dendrograms. On the whole, satDNA features (restriction pattern, sequence variation, fluorescent in situ hybridization (FISH)), allozymes and karyology support a relatively recent origin of B. whitei. Our investigations on unisexual hybrids of Bacillus also suggested that their origin and clonal reproduction allow the occurrence of different sequence subsets of limited variability in isolated populations stemming from the hybridization focus
Rapid assessment of maturation stage and reproductive mode in centrolecytic eggs of stick insects (Phasmatodea) using DAPI stain
No full textA rapid three-step DAPI technique is proposed for detecting meiotic stages and sperm head evolution in yolky, fertilized stick insect eggs, which were difficult to analyze with other methods. Fixed eggs were freed from chorionic envelopes and stained directly in DAPI/PBS solution. After rinsing, eggs were singly squashed in a drop of mounting buffer and examined under a microscope with incident fluorescent illumination. The method was almost uniformly successful, and direct observation of nuclear structures, coupled with fluorometry, allowed easy recognition of bivalents, diads, pronuclei and their DNA content. The DAPI method proposed here appears particularly helpful for investigating unusual reproductive modes in eggs with large amounts of yolk. © 1992 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted
A nonmonotone semismooth inexact Newton method
No full textIn this work we propose a variant of the inexact Newton method for the solution of semismooth nonlinear systems of equations. We introduce a nonmonotone scheme, which couples the inexact features with the nonmonotone strategies. For the nonmonotone scheme, we present the convergence theorems. Finally, we show how we can apply these strategies in the variational inequalities context and we present some numerical examples
Allozymic and cytological evidence for hemiclonal, all‐paternal, and mosaic offspring of the hybridogenetic stick insect Bacillus rossius‐grandii grandii
No full textBacillus hybridogens represent the first example of hemiclonal reproduction among invertebrates and they were analyzed in the present study for comparison with vertebrate hybridogenetic complexes (Poeciliopsis and Rana). Experimental crosses of southeastern hybridogenetic B. rossius‐grandii grandii females with both the parental species and hybrid males were carried out to test the reproductive potential of hybridogens. Allozyme and chromosome markers indicated that hemiclonal females actually reproduced in a variety of ways. Hybridogenesis was the most common and the rossius unassorted haploset was passed to the progeny while the grandii haploset was discarded during meiosis and replaced by that of the fathering male. Andro‐genesis, which was much rarer, led to the production of two types of all‐paternal offspring: unipa‐rental progeny of both sexes and all‐male progeny with the same allozyme and chromosome structure as their father. Mosaics of two kinds were also produced: the “h + h” type, with two different hybridogenetic cell lines; and the “h + a” type, with one hybridogenetic and one androgenetic cell line. Mosaics evolved into intersexes when their cell lines were sexually discordant. Among the evolutionary outcomes of this wide array of reproductive ways seems to be a shift from hemiclonality to clonality. This possibility is supported by the well‐established parthenogen B. whitei and the occasional production of rare invariant specimens from the northwestern hybridogenetic form B. rossius‐grandii benazzii. Bacillus and other unisexuals provide evidence of numerous reproductive interactions between hybrids and their bisexual relatives: the occurrence of such interactions may play an important role in the persistence, diversification and evolution of unisexuals. © 1995 Wiley‐Liss, Inc. Copyright © 1995 Wiley‐Liss, Inc., A Wiley Compan
C-banding, ag-nor localization and chromosomal repatterning in sardinian bacillus atticus (Insecta, phasmatodea)
No full textAll demcs of Sardinian Bacillus atticus share a 2n = 33 karyotype which is fully comparable to the standard one (2n = 34), except for a Robertsonian fusion of the two chromosomes of pair 11. Furthermore, the San Vito population shows a cytotype with 34 elements, derived from the common 33-chromosomes with the addition of an unusual, all-heterochromatic large acrocentric one. C-banding and silver staining have revealed a rather constant heterocromatin and NOR localizations in all samples; this suggests a most likely derivation of all Sardinian demes from common founder(s). The cytogenetic degree of differentiation of Sardinian B. atticus from other conspecific Italian demes, and from the bisexual B. grandii, is discussed. No cytogenetic evidence supporting the repeatedly stated hybrid structure of B. atticus is revealed from karyological studies, and the hybrid origin of B. atticus can be maintained on genetical grounds only. It is also suggested that the odd chromosome in the San Vito cytotype derives from a fission of a large satellite with the gain of a centromeric function. Since a paleointrogression of B. atticus genes into syntopic B. rossius had been evidenced in the Sardinian B. atticus range, the role of the 11–11 Robertsonian fusion in the hybrid zone is taken into account. © 1991, Taylor & Francis Group, LLC
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