RESUMEN
Sex chromosomes have evolved many times from morphologically identical autosome pairs, most often presenting several recombination suppression events, followed by accumulation of repetitive DNA sequences. In Orthoptera, most species have an X0â sex chromosome system. However, in the subfamily Melanoplinae, derived variants of neo-sex chromosomes (neo-XYâ or neo-X1X2Yâ) emerged several times. Here, we examined the differentiation of neo-sex chromosomes in a Melanoplinae species with a neo-XYâ/XXâ system, Ronderosia bergi, using several approaches: (i) classical cytogenetic analysis, (ii) mapping via fluorescent in situ hybridization of some selected repetitive DNA sequences and microdissected sex chromosomes, and (iii) immunolocalization of distinct histone modifications. The microdissected sex chromosomes were also used as sources for Polymerase chain reaction (PCR) amplification of RNA-coding multigene families, to study variants related to the sex chromosomes. Our data suggest that the R. bergi neo-Y has become differentiated after its formation by a Robertsonian translocation and inversions, and has accumulated repetitive DNA sequences. Interestingly, the ex autosomes incorporated into the neo-sex chromosomes retain some autosomal post-translational histone modifications, at least in metaphase I, suggesting that the establishment of functional modifications in neo-sex chromosomes is slower than their sequence differentiation.
Asunto(s)
Evolución Molecular , Saltamontes/genética , Cromosomas Sexuales , Animales , Femenino , Hibridación Fluorescente in Situ , Cariotipificación , Masculino , FilogeniaRESUMEN
Salinomys delicatus is considered a rare species due to its restricted and patchy distribution, poor records and low abundances. It is also the phyllotine with the lowest known diploid chromosome number (2n = 18), however its sex chromosome system has never been described. Here, we studied the chromosomes of six females and three males with bands G, C, DAPI/CMA(3) and meiosis. In males, the chromosome number was 2n = 19, with one large metacentric X-chromosome and two medium-sized acrocentrics absent in females. The karyotype of females was the same as previously described (2n = 18, FN = 32), with X-chromosomes being metacentric and the largest elements of the complement. In males, the two acrocentrics and the large metacentric form a trivalent in meiotic prophase. This indicates that S. delicatus has XY(1)Y(2) sex chromosomes, which is confirmed by G and DAPI bands. Constitutive heterochromatin (CH) is restricted to small pericentromeric blocks in all chromosomes. The X-chromosome shows the largest block of centromeric CH, which could favor the establishment of this X-autosome translocation. This sex chromosome system is rare in mammals and, compared with other phyllotine rodents, S. delicatus seems to have undergone a major chromosome restructuring during its karyotypic evolution.
Asunto(s)
Arvicolinae/genética , Cromosoma X , Cromosoma Y , Animales , Bandeo Cromosómico , Femenino , Cariotipo , Masculino , Meiosis , MitosisRESUMEN
Geographic body size variation was analysed in males and females of 19 populations of the South American grasshopper Dichroplus vittatus Bruner spanning 20 degrees of latitude and 2700 m of altitude. Using mean and maximum body length of each sex and factors obtained from principal components analyses of six morphometric linear characters it was shown that D. vittatus followed the converse to Bergmann's rule latitudinally but not altitudinally where no significant trends were observed. For males, variability of body size increased with latitude but not with altitude. Both types of trends were significantly correlated with mean annual temperature and minimum annual temperature (positive correlations), and two estimators of seasonality, the coefficients of variation of mean annual temperature (negative) and mean annual precipitation (positive). Some allometric relationships also showed geographic variation. It is suggested that the observed decrease in size with latitude together with the increase in morphological variability is a consequence of a number of factors: the shortening of the growing season southwards; the increasing seasonality and climatic unpredictability; and the fact that the species exhibits protandry which contributes to smaller and more variable size in males and smaller but more constant body size in females.
Asunto(s)
Tamaño Corporal/fisiología , Saltamontes/fisiología , Animales , Biometría , Clima , Femenino , Geografía , Masculino , Factores Sexuales , América del SurRESUMEN
Assortment-distortion with respect to the X and NOR activity of a rare mitotically stable B chromosome (B(N)), was examined in 16 males of Dichroplus pratensis (Acrididae: Melanoplinae) from Argentine populations. In 1B individuals, the X and B associate preferentially during prophase I reaching a maximum level of association at zygotene. Frequency of X/B association remains relatively high up to diplotene-diakinesis and decreases steeply towards metaphase I. The percent X/B association at each stage is positively influenced by association at the previous stage, and interindividual variability in X/B association decreases as the frequency of association increases. Both chromosomes tended to preferentially orientate toward the same pole at MI (mean ratio of 16 individuals, 1.50:1) which determined an excess of XB and 00 second spermatocytes over X0 and 0B ones (1.39:1). No significant differences occurred between the MI, AI and MII assortment ratios. Fluorescent in situ hybridisation (FISH) confirmed that the B chromosome carries ribosomal genes and helped to establish that, during spermiogenesis, both the B and the normal NOR-bearing chromosome (S8) are clustered near the centriole adjunct region of spermatids. However, FISH failed to reveal the existence of inactive ribosomal cistrons in the X chromosome, as previously suggested, thus providing no support to a simple origin of the B from the X.
Asunto(s)
Cromosomas/genética , Genes/genética , Saltamontes/genética , Hibridación Fluorescente in Situ/métodos , Ribosomas/genética , Cromosoma X/genética , Animales , Inestabilidad Cromosómica/genética , ADN Ribosómico/genética , Femenino , Masculino , Meiosis/genética , Profase Meiótica I/genética , Región Organizadora del Nucléolo/genética , Huso Acromático/genéticaRESUMEN
We tested the centromeric drive theory of karyotypic evolution in the grasshopper Dichroplus pratensis, which is simultaneously polymorphic for eight Robertsonian fusions and two classes of B chromosomes. A logistic regression analysis performed on 53 natural populations from Argentina revealed that B chromosomes are more probably found in populations with a higher proportion of acrocentric chromosomes, as the theory predicts. Furthermore, frequencies of B-carrying individuals are significantly negatively correlated with the mean frequency of different Robertsonian fusions per individual. No significant correlations between presence/absence or frequency of Bs, and latitude or altitude of the sampled populations, were found. We thus provide the first intraspecific evidence supporting the centromeric drive theory in relation to the establishment of B chromosomes in natural populations.
Asunto(s)
Centrómero/genética , Aberraciones Cromosómicas , Cromosomas/genética , Saltamontes/genética , Animales , Femenino , Meiosis/genéticaRESUMEN
We examined, through allozyme electrophoresis, the genetic structure of populations of the acridid grasshopper Dichroplus pratensis from two chromosomal races (Northern and Southern) and their hybrid zone in Argentina. No fixed alleles for any particular race were found, although genetic differentiation among parental races was significant (0 = 0.044, 95% CI: 0.004-0.068). Hybrid populations are genetically more similar to the Southern race (0 = 0.008, 95% CI: -0.005-0.018) than to Northern ones (0 = 0.018, 95% CI: 0.002-0.030). Differential viability or fertility of hybrids, or asymmetry in mating preferences in favour of one particular cross would cause a higher proportion of matings between hybrid individuals and those from the Southern race. This would explain the high genetic similarity between those groups, in spite of their geographical vicinity with northern race populations.
Asunto(s)
Variación Genética , Genética de Población , Saltamontes/genética , Hibridación Genética , Animales , Argentina , Análisis por Conglomerados , Electroforesis , Frecuencia de los Genes , Geografía , Isoenzimas , Reproducción/fisiologíaRESUMEN
The South American acridid grasshopper Dichroplus pratensis (Melanoplinae) is polymorphic and polytypic for a system of Robertsonian fusions across most of its distribution range. Several chromosomal races and hybrid zones have been identified. Since the fusions exert profound inter- and intrachromosomal effects on recombination, it has been proposed that these fusion systems could preserve sets of adaptive gene combinations in central (ecologically optimal) populations, and that marginal populations would show lower levels of chromosomal polymorphism and thus, increased recombination potential to cope with the harsher conditions of marginal habitats. In this paper, we describe a series of central and marginal populations which display typical features of each type and also, identify for the first time completely standard telocentric marginal populations at the southernmost and northernmost limits of the species distribution and show, as expected, the highest recombination indices. However, this modification of chiasma patterns, is only true of the male sex since females of both standard and Robertsonian populations show low chiasma frequencies. The hypothesis that higher levels of recombination would be adaptive in marginal populations is reinforced by the fact that the latter also show unusually high frequencies of B chromosomes some of which increase mean cell chiasma frequency.
Asunto(s)
Saltamontes/clasificación , Saltamontes/genética , Animales , Argentina , Femenino , Geografía , Cariotipificación , Masculino , Meiosis , Modelos Biológicos , Análisis de Regresión , UruguayRESUMEN
Dichroplus pratensis has a complex system of Robertsonian rearrangements with central-marginal distribution; marginal populations are standard telocentric. Standard bivalents show a proximal-distal chiasma pattern in both sexes. In Robertsonian individuals a redistribution of chiasmata occurs: proximal chiasmata are suppressed in fusion trivalents and bivalents which usually display a single distal chiasma per chromosome arm. In this paper we studied the synaptic patterns of homologous chromosomes at prophase I of different Robertsonian status in order to find a mechanistic explanation for the observed phenomenon of redistribution of chiasmata. Synaptonemal complexes of males with different karyotypes were analysed by transmission electron microscopy in surface-spread preparations. The study of zygotene and early pachytene nuclei revealed that in the former, pericentromeric regions are the last to synapse in Robertsonian trivalents and bivalents and normally remain asynaptic at pachytene in the case of trivalents, but complete pairing in bivalents. Telocentric (standard) bivalents usually show complete synapsis at pachytene, but different degrees of interstitial asynapsis during zygotene, suggesting that synapsis starts in opposite (centromeric and distal) ends. The sequential nature of synapsis in the three types of configuration is directly related to their patterns of chiasma localisation at diplotene-metaphase I, and strongly supports our previous idea that Rb fusions instantly produce a redistribution of chiasmata towards chromosome ends by reducing the early pairing regions (which pair first, remain paired longer and thus would have a higher probability of forming chiasmata) from four to two (independently of the heterozygous or homozygous status of the fusion). Pericentromeric regions would pair the last, thus chiasma formation is strongly reduced in these areas contrary to what occurs in telocentric bivalents.
Asunto(s)
Emparejamiento Cromosómico , Cromosomas/ultraestructura , Intercambio Genético , Saltamontes/genética , Animales , Heterocigoto , Homocigoto , Cariotipificación , Masculino , Meiosis , Metafase , Testículo/ultraestructuraRESUMEN
The origin of neo-XY sex systems in Acrididae is usually explained through an X-autosome centric fusion, and the behaviour of the neo-sex chromosomes has been solely studied in males. In this paper we analysed male and female Dichroplus vittatus. The karyotype comprises 2n = 20 chromosomes including 9 pairs of autosomes and a sex chromosome pair that includes a large metacentric neo-X and a small telocentric neo-Y. We compared the meiotic behaviour of the sex bivalent between both sexes. Mean cell autosomal chiasma frequency was low in both sexes and slightly but significantly higher in males than in females. Chiasma frequency of females increased significantly when the sex-bivalent was included. Chiasma distribution was basically distal in both sexes. Behaviour of the neo-XY pair is complex as a priori suggested by its structure, which was analysed in mitosis and meiosis of diploid and polyploid cells. During meiosis, orientation of the neo-XY is highly irregular; only 21% of the metaphase I spermatocytes show standard orientation. In the rest of cells, the alternate or simultaneous activity of an extra kinetochore in the distal end of the short arm (XL) of the neo-X, determined unusual MI orientations and a high frequency of non-disjunction and lagging of the sex-chromosomes. In females, the neo-XX bivalent had a more regular behaviour but showed 17% asynapsis in the XL arm which, in those cases orientated its distal ends towards opposite spindle poles suggesting, again, the activity of a second kinetochore. The dicentric nature and the unstable meiotic behaviour of the sex neo-chromosomes of D. vittatus suggest a recent origin of the sex determination mechanism, with presumable adaptive advantages which could compensate their potential negative heterosis. Our observations suggest that the origin of the neo-sex system was a tandem fusion of two original telocentric X-chromosomes followed by another tandem fusion with the small megameric bivalent and a further pericentric inversion of the neo-X. The remaining autosomal homolog resulted in the neo-Y chromosome.