Patterns and pathways of chromosome evolution within the Orthoptera

Expression Profiles of Microsatellites in Fruit Tissues of Akebia trifoliata and Development of Efficient EST-SSR Markers

Akebia trifoliata, a member of the family Lardizabalaceae, has high exploitation potential for multiple economic purposes, so genetic improvements to meet requirements for commercial demand are needed. However, this progress is largely impeded by a lack of effective selection markers. In this study, we obtained 271.49 Gb of clean transcriptomic data from 12 samples (three tissues at four developmental stages) of A. trifoliata fruit. We identified 175,604, 194,370, and 207,906 SSRs from the de novo assembled 416,363, 463,756, and 491,680 unigene sequences obtained from the flesh, seed, and rind tissues, respectively. The profile and proportion of SSR motifs expressed in each fruit tissue and developmental stage were remarkably similar, but many trinucleotide repeats had differential expression levels among different tissues or at different developmental stages. In addition, we successfully designed 16,869 functional EST-SSR primers according to the annotated unigenes. Finally, 94 and 72 primer pairs out of 100 randomly selected primer pairs produced clear bands and polymorphic bands, respectively. These results were also used to elucidate the expression profiles of different tissues at various stages. Additionally, we provided a set of effective, polymorphic, and reliable EST-SSR markers sufficient for accelerating the discovery of metabolic and pathway-specific functional genes for genetic improvement and increased commercial productivity.

Micro-evolution in grasshoppers mediated by polymorphic Robertsonian translocations

This review focuses on grasshoppers that are polymorphic for Robertsonian translocations because in these organisms the clarity of meiotic figures allows the study of both chiasma distribution and the orientation of trivalents and multivalents in metaphase I. Only five species of such grasshoppers were found in the literature, and all of them were from the New World: Oedaleonotus enigma (Scudder) (Orthoptera: Acrididae), Leptysma argentina Bruner, Dichroplus pratensis Bruner, Sinipta dalmani Stål, and Cornops aquaticum Bruner. A general feature of these species (except O. enigma) is that fusion carriers suffer a marked reduction of proximal and interstitial (with respect to the centromere) chiasma frequency; this fact, along with the reduction in the number of linkage groups with the consequent loss of independent segregation, produces a marked decrease of recombination in fusion carriers. This reduction in recombination has led to the conclusion that Robertsonian polymorphic grasshopper species share some properties with inversion polymorphic species of Drosophila, such as the central-marginal pattern (marginal populations are monomorphic, central populations are highly polymorphic). This pattern might be present in D. pratensis, which is certainly the most complex Robertsonian polymorphism system in the present study. However, L. argentina and C. aquaticum do not display this pattern. This issue is open to further research. Since C. aquaticum is soon to be released in South Africa as a biological control, the latitudinal pattern found in South America may repeat there. This experiment's outcome is open and deserves to be followed.

The paradoxical spread of a new Y chromosome - a novel explanation

Hybrid zones provide one of the more prickly challenges to the cherished biological species concept, which is based on reproductive isolation between species, because some hybrid zones occur in areas where recognised species come together and mate to produce viable progeny. By contrast, hybrid zones have repeatedly provided outstanding material for the study of evolution: they allow us to study ecological and genomic differences between recently diverged taxa; and they bring into sharp focus the interplay between migration, selection, recombination and drift. A new theoretical analysis now demonstrates that hybrid-zone dynamics provide a counterintuitive explanation for how a new chromosomal sex-determination system can evolve and spread across the landscape, displacing the ancestral system, even though it would be eliminated by natural selection in a single isolated population. This work thus provides a provocative resolution to a conundrum that has puzzled cytogeneticists and evolutionary biologists for years.

The inexorable spread of a newly arisen neo-Y chromosome

A newly arisen Y-chromosome can become established in one part of a species range by genetic drift or through the effects of selection on sexually antagonistic alleles. However, it is difficult to explain why it should then spread throughout the species range after this initial episode. As it spreads into new populations, it will actually enter females. It would then be expected to perform poorly since it will have been shaped by the selective regime of the male-only environment from which it came. We address this problem using computer models of hybrid zone dynamics where a neo-XY chromosomal race meets the ancestral karyotype. Our models consider that the neo-Y was established by the fusion of an autosome with the ancestral X-chromosome (thereby creating the Y and the ‘fused X’). Our principal finding is that sexually antagonistic effects of the Y induce indirect selection in favour of the fused X-chromosomes, causing their spread. The Y-chromosome can then spread, protected behind the advancing shield of the fused X distribution. This mode of spread provides a robust explanation of how newly arisen Y-chromosomes can spread. A Y-chromosome would be expected to accumulate mutations that would cause it to be selected against when it is a rare newly arrived migrant. The Y can spread, nevertheless, because of the indirect selection induced by gene flow (which can only be observed in models comprising multiple populations). These results suggest a fundamental re-evaluation of sex-chromosome hybrid zones. The well-understood evolutionary events that initiate the Y-chromosome's degeneration will actually fuel its range expansion.

Comparisons between related species have shown that, over evolutionary time scales, Y-chromosomes tend to degenerate and can be completely lost. How then can we explain the persistence of Y-chromosomes to the present? One possibility is that losses are counter-balanced by the origin of new Y chromosomes, which then spread throughout the species in which they have arisen. The first of these two processes, the generation of new Y chromsomes, is more readily understood: it can occur if an autosome (a non sex chromosome) fuses with an X chromosome. This form might become established in one locality. However, its subsequent geographic spread has been more challenging to explain. Problems arise if gene flow carries them to another part of the species range. Crosses can then occur which introduce the new Y chromosome into females, who are expected to suffer reduced fitness. The new sex chromosomes are therefore selected against when they are in the minority. We use simulations to show that they can nevertheless spread, if they meet the ancestral forms at a front so the chromosomes intermingle in a hybrid zone. Paradoxically, the degeneration of the Y will actually intensify selection, thereby speeding its spread.

Heavy metal accumulation, heat shock protein expression and cytogenetic changes in Tetrix tenuicornis (L.) (Tetrigidae, Orthoptera) from polluted areas

The orthopteran insect Tetrix tenuicornis, collected from polluted and unpolluted areas, was used to study heavy metal accumulation and its impact on stress protein levels and on changes in the number and morphology of chromosomes in mitotic and meiotic cells. During two consecutive years, insects were collected from polluted areas of zinc-lead mine spoils near Bolesław (Poland) and from unpolluted areas near Busko and Staszów (Poland). T. tenuicornis from the polluted area showed 1.5, 4.03, 4.32 and 41.73 times higher concentrations of copper (Cu), zinc (Zn), lead (Pb) and cadmium (Cd), respectively, than insects of the same species collected from unpolluted areas. Insects exposed to heavy metals showed only small changes, and rather a decrease in the concentration of constitutive and inducible heat shock proteins Hsp70, the level of which increases under stress conditions. A cytogenetic study of T. tenuicornis revealed intra-population anomalies in chromosome number and morphology in mitotic and meiotic cells and the presence of an additional B chromosome in germinal cells. In 50% of females collected from polluted areas, mosaic oogonial mitotic chromosome sets and diploid, hypo- or hypertetraploid, tetraploid, and octoploid chromosome numbers were detected. In turn, 14.6% of males showed a heterozygous deficiency of chromatin in L2 and M3 bivalents in addition to the presence of B chromosomes.

Evolution of muntjac DNA

The extent of nuclear single-copy DNA divergence between Muntiacus reevesi and Muntiacus muntjak vaginalis (Cervidae), a species pair showing extreme karyotype differences but striking morphological similarity, is 2%, as judged from the thermal stability of interspecific DNA-DNA hybrids. A comparison of the total nuclear DNA reassociation kinetics of the two species indicates a reduction of lowly repetitive sequences in M. m. vaginalis.

Karyotypic fission theory and the evolution of old world monkeys and apes

The karyotypes of living catarrhines are correlated with the current concepts of their fossil record and systematic classification. A phylogeny, beginning at the base of the Oligocene, for those animals and their chromosome numbers is presented. Todd's (1970) theory of karyotypic fissioning is applied to this case - three fissioning events are hypothesized. A late Eocene event (the primary catarrhine fissioning) is hypothesized to underlie the diversification of the infraorder Catarrhini into its extant families, the second fissioning underlies the radiation of the pongidae/Hominidae in the Miocene and the third accounts for the high chromosome numbers (54 - 72) and the Neogene(Miocene-Pliocene-Pleistocene) radiation of members of the genus Cercopithecus. Published catarrhine chromosome data, including that for "marked" chromosomes (those with a large achromatic region that is the site for ribosomal RNA genes) are tabulated and analysed. The ancestral X chromosome is always retained in the unfissioned metacentric state. The Pongidae/Hominidae have 15 pairs of mediocentric chromosomes that survived the second fissioning whereas the other chromosomes (besides the X) are thought to be fission-derived acrocentrics. Both the detailed karyology and the trend from low to high numbers is best interpreted to support Todd's concept of adaptive radiations correlated with karyotypic fissioning in ancestral populations.

Centric fission of chromosome no. 4 in the mother of two patients with trisomy 4p

Centric fission of chromosome No. 4 was found in the healthy mother of two children with trisomy 4p. The two telocentrics derived are stable and show no evidence of fusing again. The 2:3 ratio of unbalanced offsprings born to the proposita indicates that, contrary to the evidence emerging from studies in other species, the risk for the production of unbalanced gametes is high.

Achiasmate meiosis and centromere shift in Eusimulium aureum (Diptera-Simuliidae)

Light and electronmicroscope studies indicate that Eusimulium aureum (n equals 2 metacentrics) is male-achiasmate. Supporting evidence for achiasmate meiosis includes lack of recombination between, I) widely separated differential segments of X and Y chromosomes and 2) linked autosomal inversions. In sibling C, chromosome II exists in an alternate "neoacrocentric" form interpreted as originating through the three-break shift of a small segment including centromere and nucleolar organizer. Male-achiasmate meiosis appears to be a prerequisite for the establishment of rearrangements that include large displacements of the centromere. This suggestion is supported by a correlation between achiasmate male meiosis and the fixation of gross pericentric changes in a number of other black fly species.

Somatic association of telocentric chromosomes carrying homologous centromeres in common wheat

Measurements of distances between telocentric chromosomes, either homologous or representing the opposite arms of a metacentric chromosome (complementary telocentrics), were made at metaphase in root tip cells of common wheat carrying two homologous pairs of complementary telocentrics of chromosome 1 B or 6 B (double ditelosomic 1 B or 6 B). The aim was to elucidate the relative locations of the telocentric chromosomes within the cell. The data obtained strongly suggest that all four telocentrics of chromosome 1 B or 6 B are spacially and simultaneously co-associated. In plants carrying two complementary (6 B (S) and 6 B (L)) and a non-related (5 B (L)) telocentric, only the complementary chromosomes were found to be somatically associated. It is thought, therefore, that the somatic association of chromosomes may involve more than two chromosomes in the same association and, since complementary telocentrics are as much associated as homologous, that the homology between centromeres (probably the only homologous region that exists between complementary telocentrics) is a very important condition for somatic association of chromosomes. The spacial arrangement of chromosomes was studied at anaphase and prophase and the polar orientation of chromosomes at prophase was found to resemble anaphase orientation. This was taken as good evidence for the maintenance of the chromosome arrangement - the Rabl orientation - and of the peripheral location of the centromere and its association with the nuclear membrane. Within this general arrangement homologous telocentric chromosomes were frequently seen to have their centromeres associated or directed towards each other. The role of the centromere in somatic association as a spindle fibre attachment and chromosome binder is discussed. It is suggested that for non-homologous chromosomes to become associated in root tips, the only requirement needed should be the homology of centromeres such as exists between complementary telocentrics, or, as a possible alternative, common repeated sequences of DNA molecules around the centromere region.

CHROMOSOMAL POLYMORPHISM IN NORTH AMERICAN PISSODES WEEVILS: STRUCTURAL ISOMERISM

Pissodes approximatus and P. canadensis, two taxa named by Hopkins, are shown to constitute a hybrid complex that forms a numerical-morphological karyocline between the southeast of North America and the Yukon. The two A metacentrics and the two B submetacentrics in the SE (predominantly 2n = 30) are each sequentially replaced by twin acrocentrics, aa and bb, in the NW (mainly 2n = 34). The two b pairs, of markedly different length, are segmentally interchanged in the SE relative to those in the NW, the distal interchanged segments being equal. The two Bs, derived by fusion of alternative acrocentrics, simulate pericentrically inverted, complementary submetacentrics. Introgression of the four structurally different types, commonest at the centre of the karyocline, results in 10 distinct B: bb karyotypes of which only 6 appear different; the other 4 are mirror images, or isomers.The intermediate karyotypes have been synthesized by crossing different AB and aabb taxa; they implicate two pairs of isomeric species in the origin of the polymorphic complex. Of these, one AB species with 'standard' Bs and two aabb species with relatively interchanged bs are extant. The fourth, a species with inverted Bs, may be extinct, have merely escaped detection, or else is completely introgressed.Chiasma formation is near perfect in natural polyisomorphs and almost equally so in synthetics. In fact, in both bb/bb heterozygotes the maximum number of associations per cell, normally 16 (the XY pair is achiasmate), is increased by the formation of anomalous quadruple configurations resulting from extra unions at the centric ends of the bs. If chiasmate and situated in minute arms, these unions far exceed expectation on a length-frequency basis, for such arms cannot be resolved in colchicined gonial metaphases. Whether the extra unions are truly chiasmate, as believed, or not, they nevertheless greatly influence the first metaphase disjunctional arrangement of the multiples and cause production of about 80% unbalanced gametes vs. the 50% expected of random assortment, thus generating a genetic isolating mechanism. Such acrocentric heterozygotes, though carrying a heavy segregational-sterility load, persist by recurrent SE-NW introgressive hybridization.