Phylogeographic patterns in Drosophila montana.

The Drosophila virilis species group offers valuable opportunities for studying the roles of chromosomal re-arrangements and mating signals in speciation. The 13 species are divided into two subgroups, the montana and virilis 'phylads'. There is greater differentiation among species within the montana phylad in both karyotype and acoustic signals than exists among members of the virilis phylad. Drosophila montana is a divergent species which is included in the montana phylad. Here, we analyse the phylogeography of D. montana to provide a framework for understanding divergence of acoustic signals among populations. We analysed mitochondrial sequences corresponding to the cytochrome oxidase I and cytochrome oxidase II genes, as well as 16 microsatellite loci, from 108 lines of D. montana covering most of the species' range. The species shows a clear genetic differentiation between North American and Scandinavian populations. Microsatellite allele frequencies and mitochondrial DNA haplotypes gave significant FST values between populations from Canada, USA and Finland. A Bayesian analysis of population structure based on the microsatellite frequencies showed four genetically distinct groups, corresponding to these three populations plus a small sample from Japan. A network based on mitochondrial haplotypes showed two Finnish clades of very different shape and variability, and another clade with all sequences from North America and Japan. All D. montana populations showed evidence of demographic expansion but the patterns inferred by coalescent analysis differed between populations. The divergence times between Scandinavian and North American clades were estimated to range from 450,000 to 900,000 years with populations in Canada and the USA possibly representing descendants of different refugial populations. Long-term separation of D. montana populations could have provided the opportunity for differentiation observed in male signal traits, especially carrier frequency of the song, but relaxation of sexual selection during population expansion may have been necessary.

African matrilineages in American Creole cattle: evidence of two independent continental sources.

In order to clarify the historical origin and phylogeographic affinities of Creole cattle matrilineages throughout the American continent, we analysed published D-loop mtDNA sequences (n = 454) from Creole, Iberian and African cattle breeds. The Western European T3 haplogroup was the most common in American Creole cattle (63.6%), followed by the African T1 (32.4%) and the Near Eastern T2 haplogroups (4%). None of the sequences were found in Bos indicus types. Within the African T1 haplogroup there were two subclades, T1a and T1*, whose geographic distribution in America was clearly disjointed. T1a is a highly divergent clade originally reported for Creole cattle from Brazil and the Lesser Antilles, but whose geographic distribution in Africa remains unknown. In contrast, lineages attributable to T1* are restricted in America to the region colonized by the Spaniards. We propose a new hypothesis for the origins of Creole cattle that summarizes all previously published historical and genetic data. While the African T1* fraction in Creole cattle may have arrived in America through the Iberian breeds, the divergent T1a lineages may have been introduced by Portuguese and other European crowns from some unknown, not-yet-sampled African location. Additional molecular studies will be required for pinpointing the specific African regional source.

African and European mitochondrial haplotypes in South American Creole cattle.

South American Creole cattle are direct descendants of the animals brought to the New World by the Spanish and Portuguese during the 16th century. A portion of the mitochondrial D-loop was sequenced in 36 animals from five Creole cattle populations in Argentina and four in Bolivia. Individuals belonging to the potentially ancestral Spanish breed Retinta were also analysed. Sequence comparisons revealed three main groups: two with the characteristics of European breeds and a third showing the transitions representative of the African taurine breeds. The African sequences were found in two populations from Argentina and three populations from Bolivia, whose only connections go back to colonial times. The most probable explanation for the finding is that animals could have been moved from Africa to Spain during the long-lasting Arabian occupation that started in the seventh century, and from the Iberian Peninsula to America eight centuries later. However, since African haplotypes were not found in the Spanish sample, the possibility of cattle transported directly from Africa cannot be disregarded.

Molecular analysis of populations of Ctenomys (Caviomorpha, Rodentia) with high karyotypic variability.

The tuco-tucos (Ctenomys) are South American subterranean rodents that are some of the most chromosomally variable of all mammals. In this study we focus on Ctenomys of the "Corrientes species group" from that Argentine province and consisting of C. dorbignyi, C. perrensi, C. roigi and unnamed populations (Ctenomys sp.). A diploid range of 41-70 has been demonstrated for these taxa with multiple Robertsonian changes, pericentric inversions, heterochromatic modifications and other chromosomal rearrangements. To analyse the molecular variation in the Corrientes group, a 402 base pair fragment of mitochondrial cytochrome b was sequenced in 75 individuals from 26 populations. This generated 15 different haplotypes which were subjected to phylogenetic analysis. The different species within the Corrientes group failed to form monophyletic groups in the phylogenetic trees we generated and the divergences between haplotypes were low. Therefore, these molecular data demonstrate the recent subdivision of the Corrientes group with a substantial accumulation of chromosomal differences. Through our new chromosomal data and cytochrome b studies, we are able to subdivide the Ctenomys sp. populations into two groups and hypothesize on their relationships with the described Corrientes group species. Further studies are needed to establish the basis of the phenomenal chromosomal variation in this group.

Phylogenetic relationships of Argentinean Creole horses and other South American and Spanish breeds inferred from mitochondrial DNA sequences.

South American horses constitute a direct remnant of the Iberian horses brought to the New World by the Spanish conquerors. The source of the original horses was Spain, and it is generally assumed that the animals belonged to the Andalusian, Spanish Celtic, Barb or Arabian breeds. In order to establish the relationship between Argentinean and Spanish horses, a portion of the mitochondrial D-loop of 104 animals belonging to nine South American and Spanish breeds was analysed using SSCP and DNA sequencing. The variability found both within and between breeds was very high. There were 61 polymorphic positions, representing 16% of the total sequence obtained. The mean divergence between a pair of sequences was 2.8%. Argentinean Creole horses shared two haplotypes with the Peruvian Paso from Argentina, and the commonest haplotype of the Creole horses is identical to one of the Andalusian horses. Even when there was substantial subdivision between breeds with highly significant Wright's Fixation Index (FST), the parsimony and distance-based phylogenetic analyses failed to show monophyletic groups and there was no clear relationship in the trees between the South American and any of the other horses analysed. Although this result could be interpreted as mixed ancestry of the South American breeds with respect to the Spanish breeds, it is probably indicating the retention of very ancient maternal lineages in the breeds analysed.

Male-mediated introgression of Bos indicus genes into Argentine and Bolivian Creole cattle breeds.

The geographic distribution and frequency of Bos taurus and Bos indicus Y chromosome haplotypes amongst Argentine and Bolivian Creole cattle breeds were studied, using cytogenetic and molecular genetic techniques. A complete correspondence between Y chromosome morphology and the haplotype of the Y-linked microsatellite marker INRA 124 was found in all males examined. The taurine and indicine haplotypes were detected in 85.7 and 14.3% of the males studied, respectively, although these frequencies varied amongst the different breeds examined. The geographic distribution of this polymorphism suggests a pattern of zebu introgression in South America. The highest frequencies of the Zebu Y-chromosome are found in Brazilian populations (43-90%), in the eastern part of the continent, while it is absent in the southernmost breeds from Uruguay and Argentina. Bolivian breeds, at the centre of the continent, exhibit intermediate values (17-41%). This east/west and north/south gradient of male Zebu introgression could be explained by historical events and environmental factors.

Multiple nuclear pseudogenes of mitochondrial cytochrome b in Ctenomys (Caviomorpha, rodentia) with either great similarity to or high divergence from the true mitochondrial sequence.

A fragment of the mitochondrial cytochrome b gene was studied in 13 species of the South American fossorial rodent Ctenomys using PCR with 'universal' primers and DNA sequencing after cloning. Five different groups of sequences were found, one of which corresponds to the functional mitochondrial gene (mt). The other four groups (A, B, C and D) were believed to be nuclear pseudogenes. Sequences A-C were highly divergent from the mt sequences and included substitutions, deletions and insertions such that they could not possibly have coded a functional protein. They all shared a common insertion between positions 15055 and 15056 suggestive of a common origin, although the A, B and C sequences otherwise differed greatly from each other. The D sequences also could not have been functional on the basis of nucleotide sequence, but the differences with the mt sequences were far more subtle and in a more limited study the D sequences could easily have been classified as a true mtDNA sequence. It is suggested that there were two transfers of the cytochrome b gene from the mitochondrion to the nucleus; the first leading to sequences A-C and the second to the D sequence. Subsequent to transfer, a sequence of duplications within the nucleus appears to have generated the full range of pseudogenes that are observed. This study adds to other recent observations suggesting the frequent transfer of mtDNA sequences to the nucleus and reinforces the necessity of great care in interpreting PCR-generated sequences, particularly those produced with universal primers. There are now data from several species of mammals and birds relating to PCR-generated nuclear copies of cytochrome b, which we review.

Mediterranean Europe as an area of endemism for small mammals rather than a source for northwards postglacial colonization.

There is a general perception that central and northern Europe were colonized by range expansion from Mediterranean refugia at the end of the last glaciation. Data from various species support this scenario, but we question its universality. Our mitochondrial DNA studies on three widespread species of small mammal suggest that colonization may have occurred from glacial refugia in central Europe-western Asia. The haplotypes on the Mediterranean peninsulae are distinctive from those found elsewhere. Rather than contributing to the postglacial colonization of Europe, Mediterranean populations of widespread small mammals may represent long-term isolates undergoing allopatric speciation. This could explain the high endemism of small mammals associated with the Mediterranean peninsulae.

Orientation and segregation of Robertsonian trivalents in Dichroplus pratensis (Acrididae).

Pairing behavior, metaphase I orientation, and anaphase I segregation of centric fusion trivalents were studied in 26 single, 15 double, and 2 triple male fusion heterozygotes of the polymorphic South American melanopline grasshopper Dichroplus pratensis. They represent the seven different fusions and their combinations already described in different populations of the species. Our analysis showed the following: (1) pairing behavior is very regular in all trivalents; (2) frequencies of linear orientation was very low irrespective of the trivalent involved; (3) reorientation seems to occur frequently since frequencies of abnormal segregation and aneuploid second division cells were invariably lower than those of nonconvergent orientation; (4) aneuploidy and abnormal sperm production increases with increasing number of fusions; (5) chiasma frequency and localisation is relevant to trivalent orientation since trivalents with nonconvergent orientations showed proximal and interstitial chiasmata more frequently than convergently oriented ones. The results are in agreement with the hypothesis that these polymorphisms are old and stable, and confirm that for the maintenance of a balanced polymorphism, if this polymorphism is adaptive because of its consequences on recombination, position effects, etc., changes tending to stabilise trivalent orientation and segregation are central.