Marco Corti

Use of Two-Block Partial Least-Squares to Study Covariation in Shape

The relatively new two-block partial least-squares method for analyzing the covariance between two sets of variables is described and contrasted with the well-known method of canonical correlation analysis. Their statistical properties, type of answers, and visualization techniques are discussed. Examples are given to show its usefulness in comparing two sets of variables--especially when one or both of the sets of variables are shape variables from a geometric morphometric study.

Chromosomal speciation and phenotypic evolution in the house mouse

Chromosomal races of the house mouse (Mus musculus domesticus) in Valtellina and Orobian Alps (Northern Italy) are known for their very fast raciation. Here we present a study using geometric morphometrics on size and shape changes in the skull and the mandible of three races (Orobian, Upper Valtellina, Poschiavo) occurring in this area and forming a presumably monophyletic group. One of the races (Upper Valtellina) went extinct recently in an area of sympatry with the Poschiavo race, so that data on genetics (chromosomes and allozymes), behaviour and morphology were available to investigate causes of phenotypic divergence during speciation with a recent extinction event. The evaluation of partial warp scores and the uniform component shows that morphological changes have been fast and that these races can be recognized on the basis of skull shape. Patterns of evolution in shape changes were visualized by combining the chromosomal phylogeny and shape space, summarizing therefore both the phenetic and cladistic relationships. Shape changes follow the cladogenetic sequence depicted by chromosomal fusions. The examination of Procrustes distances shows that the different parts of the skull evolved at different rates after speciation, with shifts in the integration of the various structures (olfactory, auditory, feeding, visualization, etc.). Among the possible causes, aggressive behaviour was advocated for sudden changes in the shape of the skull.

Karyotype and intermale aggression in wild house mice: Ecology and speciation

Social aggression shown by male mice from two races from upper Valtellina in Italy with 2n=24 and 26 chromosomes, respectively, was contrasted in dyadic encounters. In encounters with mice of the same karyotype 2n=26 were uniformly more aggressive than their 2n=24 counterparts. When resident mice from both these races were tested against intruders of the other karyotype, 2n=26 mice again were more likely to prove aggressive and to eventually dominate their partners even when they were the intruder. Perhaps those behavioral differences account for the apparent replacing of the 2n=24 chromosome race by the 2n=26 variety in geographical areas where they overlap.

Cytogenetic analyses of chromosomal rearrangements in Mus minutoides/musculoides from North-West Zambia through mapping of the telomeric sequence (TTAGGG)n and banding techniques

Three specimens of M. minutoides/musculoides from Zambia were cytogenetically studied through G- and C-banding, DAPI staining and fluorescence in-situ hybridization (FISH) with a (TTAGGG)n telomeric sequence. Biarmed chromosomes were identified according to the current nomenclature as follows: Rb(2.7), Rb(3.12), Rb(4.5), Rb(6.8), Rb(9.16), and the sex chromosomes Rb(1.X), Rb(1.Y) and Rb(1.Xd), originated from the deleted X chromosome. One female showed the diploid number 2n=24; in the two other individuals, the Rb(9.16) occurred in a heteromorphic condition, and, accordingly, the diploid number was 2n=25. FISH showed the sites of telomeric sequences at telomeres of all the chromosomes, and in an interstitial position at the centromeres of all Robertsonian metacentrics, except one – the Rb(6.8), though the patterns of hybridization varied between chromosomes. Sex chromosome pairs, in the male and females, showed a similar C-banding pattern, but revealed clear differences after FISH. Traces of telomeric sequences were found dispersed in the whole-heterochromatic arm of the Rb(1.Xd). No visible bond between C-positive heterochromatin and telomeric sequences were detected in the other either bi- or uniarmed chromosomes, indicating that they may actually represent retained telomeres in the Robertsonian metacentrics.

Size and Shape Variation in the Mandible of the Fossorial Rodent Spalax ehrenbergi

Three-dimensional Procrustes analysis was used to study the variation of mandible size and shape in the fossorial rodent superspecies Spalax ehrenbergi across the four chromosomal species in Israel (2n = 52, 54, 58 and 60) and the Egyptian species (2n = 60). Because the animals use their incisors to dig their underground tunnel systems, we selected the mandible as a potentially rich source of information on phylogenetic and adaptive processes that characterized the evolution of the superspecies during Pleistocene and Holocene times. Eleven landmarks were recorded from the mandible in three dimensions as x, y and z coordinates. Differences in landmark position among populations and species, based on a 3D graphic visualization of the landmarks, were studied after Procrustes Generalized Least Square (GLS) fitting through uni- and multivariate statistical analysis. Sexual dimorphism was found only for size. Size also changes in a consistent pattern for species, geography, soil type and other ecological descriptors. A principal component analysis of the “shape” GLS residuals and the Mahalanobis distances between populations shows a pattern consistent with species differences in chromosomes. This favors a phylogenetic interpretation for the observed pattern of variation. However, Partial Least Squares indicate that the change is also related to geography and current ecology and is associated with the increase in diploid number, suggesting that ecological factors affected speciation.

Form changes in the sea bass, Dicentrarchus labrax (Moronidae: Teleostei), after acclimation to freshwater: an analysis using shape coordinates

SynopsisMorphological changes in the sea bass, Dicentrarchus labrax (Perciformes: Moronidae), were investigated after an experimental acclimation trial to freshwater. The sea bass is an euryhaline species occurring in the Mediterranean and west Atlantic from 30° N to 55° N. Part of the offspring of a pool of breeders was acclimated to freshwater at 9 months of age while maintaining the original stock in marine water. The effect of acclimation to freshwater over the entire form of the fish was studied through geometric morphometrics (shape coordinates). Changes in the form were shown graphically as landmark displacements though age classes 7,12, 15, 19 and 24 months and were discussed in the light of integrated growth. A significantly faster growth in freshwater was detected. Shape coordinates analyzed through multivariate statistics show that significant differences in shape arise after acclimation to freshwater. Moreover, the uniform component of shape change reveals a major effect of stretching or compression perpendicular to the body axis which is common and conservative in both trials. Differences are discussed in terms of selection and ecophenotypism.

Cytogenetics of a new cytotype of African Mus (subgenus Nannomys) minutoides (Rodentia, Muridae) from Kenya: C- and G- banding and distribution of (TTAGGG) n telomeric sequences

We present the results of a cytogenetic study on Mus (Nannomys) minutoides from Kenya by means of C- and G- banding and in-situ fluorescence hybridization (FISH) to localize the telomeric sequences. The karyotype is characterized by the occurrence of several Rb chromosomes Rb(1.X), Rb(1.Y). Rb(2.17), Rb(3.13), Rb(4.10), Rb(5.11), Rb(6.7), Rb(8.12), not previously described for this species. This finding suggests a high level of chromosomal diversification, which means it is possible to consider this cytotype as a new, well-differentiated, chromosomal lineage within the subgenus. The C-banding of the metaphases illustrated conspicuous blocks of centromeric heterochromatin at the paracentromeric regions of all telocentric chromosomes. Centromeric heterochromatin is not visible on all biarmed chromosomes. Following hybridization with telomeric probes, bright interstitial telomeric sequence (ITS) fluorescence signals are evident at the pericentromeric area of all Rb chromosomes, with the exception of Rb(2.17). Considering the localization of the C-positive heterochromatin and of the telomeric sequences, the events leading to the Kenyan cytotype from an all-telocentric condition probably included two steps: first, fusion without loss of heterochromatin and pericentromeric telomeric sequences; second, the reduction of the C-positive satellite DNA followed by the amplification of telomeric sequences in the C-negative paracentromeric region of Rb chromosomes. The presence of a single Rb(2.17) without ITS indicates possible variations of this mechanism.

Size and shape changes in the skull accompanying speciation of South American spiny rats (Rodentia: Proechimys spp.)†

Size and shape changes in the skull of South American spiny rats Proechimys spp. were investigated using geometric morphometrics. Six species were studied ( P. guairae, P. poliopus , P. trinitatis , P. canicollis , P. mincae and Proechimys sp.) using 12 populations from Venezuela and Colombia. Proechimys poliopus , P. guairae and Proechimys sp. have undergone recent speciation coupled with chromosomal rearrangements. The separate analysis of size and shape showed that the only sexually dimorphic character is size; P. mincae is the smallest species and Proechimys sp. the largest. The populations are all recognizable by their shape, and populations of the same species share a common kind of shape modification. The prevalent non-uniform nature of morphological change in the skull shows how the various structures, such as those involved with audition, feeding and olfaction, are modified differently among species. Consequently, the modification of these different structures over a long time could reflect a progressive diversification of the microniche of each species.

Shape Changes during the Growth of the Sea Bass, Dicentrarchus labrax (Teleostea: Perciformes), in Relation to Different Rearing Conditions

Thin-plate spline regression analysis is applied to sample of sea bass Dicentrarchus labrax, reared at two different salinities, i.e., marine and freshwater, in order to show shape changes and to test statistically morphological differences. All specimens were derived from the same breeding stock and were sampled at five different ages. Centroid size is used as the independent variable in the thin-plate spline regression analysis, and splines at extreme values of centroid size are computed and plotted. Differences in centroid size, for Bookstein’s uniform components (UI and U2) as well as for the pure nonuniform components of shape change are tested for significance. These analyses allow a visualization of allometry and description and testing of significance of the morphological plasticity of the sea bass. In this sense they can be valuable tools in the study of shape change during ontogeny.

Role of contact areas in chromosomal speciation of the European long-tailed house mouse (Mus musculus domesticus)

Abstract Chromosomally rearranged populations of house mouse (Mus musculus domesticus) with different diploid numbers (2n=26, 24 and 22), come into contact both inter se and with the 40-chromosome unchanged populations surrounding them, giving rise to tension zones in the sense used by Key. The evolutionary role of contact areas was emphasized as essential steps during the accumulation phase of chromosomal (Robertsonian) rearrangements as well as a key element in the chromosomal post-mating isolation. The structure and the dynamics of the hybridization zones, between 40-chromosomes and Robertsonially rearranged mouse populations, were studied and the possibility of gene flow was tested analyzing the allozymic variation; a speciation model was proposed for Mus musculus domesticus on the basis of its chromosomal intraspecific variability.