Claudio J. Bidau

The effects of Robertsonian fusions on chiasma frequency and distribution in the house mouse ( Mus musculus domesticus ) from a hybrid zone in northern Scotland

Chiasma frequency and distribution were studied in male Mus musculus domesticus from the John O’Groats–standard chromosomal hybrid zone in northern Scotland. Individuals of the John O’Groats race (2n=32; homozygous for the Robertsonian fusions 4.10, 6.13, 9.12 and 11.14) and the standard race (2n=40, all telocentric), and hybrids with various karyotypes, were examined. Chiasma frequency was significantly negatively correlated with the number of Robertsonian configurations in the meiotic cell. The decrease of chiasma frequency can be attributed to intrachromosomal effects that reduce the number of chiasmata in Robertsonian bivalents (formed in homozygotes for Robertsonian fusions) and trivalents (formed in heterozygotes). However, the reduction is more pronounced in Robertsonian bivalents and is related to a shift of chiasmata to the distal ends of the chromosome arms. A different type of repatterning occurs in trivalents where there is a significant increase in proximal and interstitial chiasmata.

Multivalents resulting from monobrachial homologies within a hybrid zone in Dichroplus pratensis (Acrididae): meiotic orientation and segregation.

Natural populations of the South American grasshopper Dichroplus pratensis differ for seven polymorphic centric fusions involving the six longest telocentric autosomes of the karyotype (L1–L6). Hence, the same telocentric is involved in more than one fusion in different populations producing metacentrics with monobrachial homologies. The present study concerns the meiotic behaviour of quadrivalents and quinquivalents formed in natural hybrids that occur in two hybrid zones between chromosomal races with monobrachial homologies. The analysis revealed that: (a) non-alternate orientation of multivalents at Metaphase I (MI) was high, ranging from 18.5 to 64.2% of the cells in 18 hybrid males. Non-alternate Prometaphase (PMI) orientation was studied in six males (two of each type of hybrid) and in all cases values were higher than in MI, which suggests reorientation between PMI and MI. (b) All hybrids showed high frequencies of aneuploid and diploid second spermatocytes which are the result of abnormal segregation and lagging of chromosomes involved in multivalent formation. A highly significant correlation exists between the frequency of abnormal MI cells per male and abnormal second division spermatocytes. In most individuals, however, the frequency of abnormal second spermatocytes was lower than that of abnormal MI, which suggests further reorientation of multivalents towards alternate orientation at MI or spermatocyte selection between both meiotic divisions, (c) The hybrids have an increased production of macrospermatids.The behaviour of the multivalents suggests that the inter-racial hybrids have their fertility moderately to severely reduced which infers the existence of post-mating reproductive isolation between races. This is discussed in relation to the maintenance and adaptive role of the fusion polymorphisms in nature.

Meiosis and the Neo-XY system of Dichroplus vittatus (Melanoplinae, Acrididae): a comparison between sexes.

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.

Distribution of chromosome frequencies within a hybrid zone of Dichroplus pratensis (Melanoplinae, Acrididae).

The widespread South American melanopline grasshopper Dichroplus pratensis is chromosomally polytypic. Seven different Robertsonian translocations between six L-autosomal pairs of the standard all-telocentric karyotype were detected; several races in Argentina are characterized for being polymorphic for one to three of these rearrangements. Within contact zones monobrachial homologies between fusion metacentrics thus occur, and hybrids (complex structural heterozygotes) are formed, in whose meiosis, quadrivalents and quinquivalents prone to non-convergent orientation and unbalanced segregation are produced. We analyse the karyotypic constitution of populations of D. pratensis from a hybrid zone between races that, although sharing the polymorphic 3/4 fusion, differ for fusions with monobrachial homologies as described above. The Sierra de la Ventana hybrid zone has certain distinctive characteristics: the frequencies and type of the fusions vary abruptly within short distances; these fusions show a mosaic pattern distribution; all the monobrachially homologous fusion metacentrics were found in high frequency. The possible origin of this hybrid zone is discussed.

Distribution of genetic variability in populations of two chromosomal races of Dichroplus pratensis (Melanoplinae, Acrididae) and their hybrid zone

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 (θ = 0.044, 95% CI: 0.004–0.068). Hybrid populations are genetically more similar to the Southern race (θ = 0.008, 95% CI: −0.005–0.018) than to Northern ones (θ = 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.

Non-random patterns of non-disjunctional orientation in trivalents of multiple Robertsonian heterozygotes of Dichroplus pratensis (Acrididae)

Metaphase I orientation of centric fusion trivalents was studied in 24 single, 19 double and 3 triple heterozygotes of Dichroplus pratensis. Different populations of this South American melanopline grasshopper are polymorphic for seven Robertsonian fusions, and the polymorphisms seem to be stable. Several cytogenetic factors involved in the orientation and segregation of the meiotic configurations such as chromosomal length, symmetry and number and position of chiasmata, have been analysed in previous works. In this paper we study another factor that is relevant in the above respect in individuals with more than one heterozygous fusion: interaction among configurations regarding orientation.Our results indicate that, when there are two or three trivalents present in the MI cell, there is an interaction in such a way that the number of metaphases in which the two or three trivalents are non-disjunctionally oriented is always significantly higher than expected under a hypothesis of independence. However, the number of cells in which all trivalents are disjunctionally oriented does not decrease significantly, so an increase of unbalanced gametes due to this factor is not expected. The stability of the polymorphisms would thus not be affected.

Rensch’s Rule in Dichroplus pratensis: A Reply to Wolak

The critique of our paper (Bidau and Martṍ 2007) by Wolak (2008) is based on two premises: 1) a misinterpretation of RenschOs rule deÞnition and 2) the statistical methodology. Regarding point 1, we do not agree. Rensch (1950) described an interspeciÞc pattern bywhich, in phylogenetically related species (he used as examples, mammals, birds, and carabid beetles), sexual size dimorphism (SSD) increases with the increase of general body size: “. . . the rule is valid that in numerous animal groups the sexual dimorphism increases with body size” (Webb and Freckleton 2007, also see Reiss 1989, p. 117). Also, in a reviewof contemporary bird literature (Anonymous 1951) the following commentary was made on the paper by Rensch (1950): “Animals of large size show relatively greater sexual dimorphism than do closely related animals of smaller size.” Interestingly, this criterion is still used in many recent papers on SSD (see below). Later, Rensch (1959) stated that “In species of birds in which the male is larger than the female, the relative sexual difference [in size] increaseswith body size. If byway of exception, the females are larger than the males, as among many species of birds of prey, the opposite correlation applies, i.e., the greater sexual difference is found in the smaller species, as cited by Wolak (2008) and Selander (1966). Rensch (1959)was probably speaking of an “exception” because, although cases of female-biased SSD were already known, he thought that “reversed” SSD was a totally different phenomenon (see below). Although no evidence of the former is provided byRensch (1959) and in a 1953 reference (cited in Rensch, 1959 and Reiss, 1989, p. 170), many studies have analyzed SSD in relation to RenschOs rule in insects and other taxa where females are larger thanmales.Again, results are consicting and not conforming to a general “rule” (Webb and Freckleton 2007). Wolak (2008) states that “RenschOs Rule is fully deÞnedas the allometryobservedwhenSSD increases with size if males are the larger sex, but when SSD decreases with size if females are the larger sex.” This is not RenschOs truly original deÞnition but that of Fairbairn (1990, 1997) and Fairbairn and Preziosi (1994). In fact, some recent papers use the original deÞnition of RenschOs rule. For example, Dale et al. (2007, p. 2971) say, “In 1950, Rensch Þrst described that in groups of related species, sexual size dimorphism is more pronounced in larger species. This widespreadand fundamental allometric relationship is now commonly referred as ORenschOs rule.O” Furthermore, althoughRenschOs Þrst proposal, the increase of SSD in cases of male-biased SSD (MBSSD), has received wide support, the decrease of SSD when the female is the larger sex (FBSSD) remains unproved (Webb and Freckleton 2007), even in birds of prey, RenschOs (1959) original example (Webb and Freckleton 2007, table 2). Other problems of deÞnition occur: in a recent paper (Blanckenhorn et al., 2007, p. 246) state that, “Rensch (1950) observed that SSD increases with body size in species where males are larger and decreases with body size in species where females are larger (recently termed RenschOs rule; Abouheif and Fairbairn 1997; Fairbairn 1997).” The former is not exactly correct: literature on SSD mentions the patternwhich is being discussed in this paper as RenschOs rule, at least since 1966. See, for example, the discussion of Selander (1966) discussion under “Body Size andSexualDimorphism,”p. 142Ð143.Also,Earhart and Johnson (1970), p. 255, in their study of owls, observe that, “This increased dimorphism in the larger species contradicts “RenschOs Rule”, and Gibbons and Lovich (1990), p. 10, in a work on sexual dimorphism of turtles, indicate that “The absence of an obvious relationship casts serious doubts on the applicability of ORenschOs RuleO, . . . ”. In our paper,we tried to establishwhetherRenschOs rule operated at an intraspeciÞc level in the grasshopper Dichroplus pratensis Bruner. IntraspeciÞc evidence of RenschOs rule in most animals is very scarce (Bidau and Martṍ 2007, 2008). We analyzed the patterns of SSD in a species showing high body size variation related to geographic factors. We chose to use the classical RenschOs rule deÞnition as considered above, because exceptions toRenschOs rule sensuFairbairn and Preziosi (1994), are very numerous (Webb and Freckleton 2007). Independently of the allometric trend that theoretically seems to be the same as shown by Fairbairn (1997), factors responsible for SSD in species with FBSSD, are probably a response to totally different evolutionary constraints. In fact, trends are not mechanisms neither processes. Furthermore, our approach to SSD in D. pratensis was obviously intraspeciÞc. IntraspeciÞc RenschOs rule has no conÞrmation whatsoever due to the lack of a reasonable number of proper studies (see Blanckenhorn et al. (2007)). Regarding the secondpoint raisedbyWolak(2008), wedoagree that type II regression is thebest approach to analyze SSD(Ranta et al. 1994; Fairbairn 1997).Our data were reanalyzed using reduced major axis regression (Bidau and Martṍ 2008); although the result was different from that reported originally (Bidau and 1 Corresponding author: Laboratorio de Biologia e Parasitologia de Mamṍferos Silvestres Reservatorios, Instituto Oswaldo Cruz, FIOCRUZ, Av. Brasil 4365, Pav. Arthur Neiva, sala 14, ManguinhosÐ 21045-900, Rio de Janeiro, RJ, Brazil. (e-mail: bidau50@gmail.com). 2 Laboratorio de Genetica Evolutiva, Universidad Nacional de Misiones, Felix de Azara 1552, 3300-Posadas, Argentina.

Meiotic behavior of Robertsonian heterozygotes in populations of Dichroplus pratensis (Acrididae) with different fusion frequencies

The meiotic behavior of Robertsonian heterozygotes of Dichroplus pratensis was analyzed in order to establish the nature of the fusion polymorphisms (stable or transient) that occur in the species. The range of fusion frequencies varies widely for each fusion studied and populations with extreme frequencies exist, which could indicate a tendency for the loss or fixation of a given rearrangement. Our results revealed that no significant correlation exists between orientation (convergent and non-convergent) in PMI and MI and fusion frequency, nor between aneuploidy and fusion frequency. Thus, orientation and segregation seem to be independent of the frequencies of the fusions, which also do not appear to affect severely the fertility of heterozygotes. Our data suggest that the polymorphisms are, at least, cytogenetically stable.

Ontogenetic allometry conservatism across five teleost orders

Geometric morphometrics were used to analyse ontogenetic trajectories in representatives of the Characiformes, Cichliformes, Cyprinodontiformes, Siluriformes, and Tetraodontiformes. It was not possible to differentiate any allometric growth patterns across groups, indicating that a phylogenetically conserved developmental pattern is widespread throughout Teleostei.