Alfredo Ruiz

The evolutionary history of Drosophila buzzatii . XIV. Larger flies mate more often in nature

Body size of wild mating males and females of the cactophilic species Drosophila buzzatii was larger and tended to be less variable than that of randomly sampled flies. The intensity of sexual selection was estimated to be 0·34 in males and 0·16 in females (average 0·25). Coefficients of rank correlation for the body size of mating pairs are not statistically different from 0, pointing out that no significant assortment for size occurs in our sample. The results can be interpreted as due to the vigour or general activity levels of larger flies which are more likely to encounter suitable mates than smaller ones, although differences in size could exist among age-classes.

Evolutionary cytogenetics of the Drosophila buzzatii species complex

The salivary gland chromosomes of 10 species in the Drosophila mulleri subgroup (repleta group) have been re-analysed. These include the eight members of the South American buzzatii and martensis clusters, previously ascribed to the mulleri complex, and the two Caribbean species D. stalkeri and D. richardsoni, previously comprising the stalkeri complex. The chief results can be summarized as follows. Inversion 3a is not present in the martensis cluster. Hence, there is no cytological link between this cluster, or the buzzatii cluster, and the rest of the mulleri complex. Accordingly, a new species complex, the buzzatii complex, is established with the two South American clusters. D. stalkeri and D. richardsoni share at least two inversions with all the species in the buzzatii and martensis clusters, and produce hybrids in interspecific crosses with many of them. This indicates a close phylogenetic relationship. Therefore, D. stalkeri and D. richardsoni are incorporated as a cluster within the newly erected buzzatii complex. A phylogenetic tree illustrating the chromosomal evolution of the buzzatii complex is presented and all the previous cytological information concerning its members is reviewed.

The evolutionary history of Drosophila buzzatii. XX. Positive phenotypic covariance between field adult fitness components and body size

In the cactophilic species Drosphila buzzatii, it is feasible to infer the action of natural selection by simultaneously sampling different life history stages in the field. During four years of research, samples of mating and non‐mating adults and pupae were taken from a natural population. The main adult fitness components, i.e., mating success, longevity, and fecundity, were recorded in relation to body size, as measured by thorax length. The age of flies was estimated by observing the developmental stage of the reproductive system. Our data showed that larger flies can outlive and outmate small flies, and that mating success is related to age. An estimate of the fitness function showed a linear increase of mating success with increasing thorax length. There was no assortative mating for this trait. We advance the hypothesis that mating success is related to the rate of encounter and courtship time through general activity, which in turn may be related to body size. A positive phenotypic correlation between thorax length and ovariole number, which is related to fecundity, was found in females emerged from wild pupae. Neither the phenotypic nor the genetic (additive) correlations between these two traits were statistically different from zero in laboratory reared females. The genetic consequences of the observed phenotypic selection on body size are discussed.

Antagonistic pleiotropic effect of second-chromosome inversions on body size and early life-history traits in Drosophila buzzatii

A simple way to think of evolutionary trade‐offs is to suppose genetic effects of opposed direction that give rise to antagonistic pleiotropy. Maintenance of additive genetic variability for fitness related characters, in association with negative correlations between these characters, may result. In the cactophilic species Drosophila buzzatii, there is evidence that second‐chromosome polymorphic inversions affect size‐related traits. Because a trade‐off between body size and larval developmental time has been reported in Drosophila, we study here whether or not these inversions also affect larva‐adult viability and developmental time. In particular, we expect that polymorphic inversions make a statistically significant contribution to the genetic correlation between body size (as measured by thorax length) and larval developmental time. This contribution is expected to be in the direction predicted by the trade‐off, namely, those flies whose karyotypes cause them to be genetically larger should also have a longer developmental time than flies with other karyotypes. Using two different experimental approaches, a statistically significant contribution of the second‐chromosome inversions to the phenotypic variances of body size and developmental time in D. buzzatii was found. Further, these inversions make a positive contribution to the total genetic correlation between the traits, as expected by the suggested trade‐off. The data do not provide evidence as to whether the genetic correlation is due to antagonistic pleiotropic gene action or to gametic disequilibrium of linked genes that affect one or both traits. The results do suggest, however, a possible explanation for the maintenance of inversion polymorphism in this species.

The Transposon Galileo Generates Natural Chromosomal Inversions in Drosophila by Ectopic Recombination

Background Transposable elements (TEs) are responsible for the generation of chromosomal inversions in several groups of organisms. However, in Drosophila and other Dipterans, where inversions are abundant both as intraspecific polymorphisms and interspecific fixed differences, the evidence for a role of TEs is scarce. Previous work revealed that the transposon Galileo was involved in the generation of two polymorphic inversions of Drosophila buzzatii. Methodology/Principal Findings To assess the impact of TEs in Drosophila chromosomal evolution and shed light on the mechanism involved, we isolated and sequenced the two breakpoints of another widespread polymorphic inversion from D. buzzatii, 2z 3. In the non inverted chromosome, the 2z 3 distal breakpoint was located between genes CG2046 and CG10326 whereas the proximal breakpoint lies between two novel genes that we have named Dlh and Mdp. In the inverted chromosome, the analysis of the breakpoint sequences revealed relatively large insertions (2,870-bp and 4,786-bp long) including two copies of the transposon Galileo (subfamily Newton), one at each breakpoint, plus several other TEs. The two Galileo copies: (i) are inserted in opposite orientation; (ii) present exchanged target site duplications; and (iii) are both chimeric. Conclusions/Significance Our observations provide the best evidence gathered so far for the role of TEs in the generation of Drosophila inversions. In addition, they show unequivocally that ectopic recombination is the causative mechanism. The fact that the three polymorphic D. buzzatii inversions investigated so far were generated by the same transposon family is remarkable and is conceivably due to Galileos unusual structure and current (or recent) transpositional activity.

THE EVOLUTIONARY HISTORY OF DROSOPHILA BUZZATII. VIII. EVIDENCE FOR ENDOCYCLIC SELECTION ACTING ON THE INVERSION POLYMORPHISM IN A NATURAL POPULATION

The pattern of selection acting in nature on the chromosomal polymorphism of the cactophilic species Drosophila buzzatii was investigated by comparing inversion and karyotypic frequencies through four different life‐cycle stages: adult males, eggs, third‐instar larvae, and immature adults. All population samples were obtained in June 1981 at an old Opuntia ficus‐indica plantation near Carboneras, Spain. The analysis rests on several assumptions which are explicitly set forth and discussed. The results, if these assumptions prove true, indicate strong directional selection for larval viability acting on the second‐chromosome karyotypes and also suggest selective differences in fecundity and longevity. Heterotic selection, however, cannot be ruled out for other fitness components such as male mating success. This kind of selection could be operating on the fourth‐chromosome polymorphism as well. Some gene arrangements showed significant and opposite changes in frequency at different parts of the life cycle, thus demonstrating endocyclic selection.

Low occurrence of gene transposition events during the evolution of the genus Drosophila.

Abstract.— The role played by gene transpositions during the evolution of eukaryotic genomes is still poorly understood and indeed has been analyzed in detail only in nematodes. In Drosophila, a limited number of transpositions have been detected by comparing the chromosomal location of genes between different species. The relative importance of gene transposition versus other types of chromosomal rearrangements, for example, inversions, has not yet been evaluated. Here, we use physical mapping to perform an extensive search for long‐distance gene transpositions and assess their impact during the evolution of the Drosophila genome. We compare the relative order of 297 molecular markers that cover 60% of the euchromatic fraction of the genome between two related Drosophila species and conclude that the frequency of gene transpositions is very low, namely one order of magnitude lower than that of nematodes. In addition, gene transpositions seem to be events almost exclusively associated with genes of repetitive nature such as the Histone gene complex (HIS‐C).

Genetic assessment of the Iberian wolf Canis lupus signatus captive breeding program

AbstractThe main goal of ex situ conservation programs is to improve the chances of long term survival of natural populations by founding and managing captive colonies that can serve as a source of individuals for future reintroductions or to reinforce existing populations. The degree in which a captive breeding program has captured the genetic diversity existing in the source wild population has seldom been evaluated. In this study we evaluate the genetic diversity in wild and captive populations of the Iberian wolf, Canis lupus signatus, in order to assess how much genetic diversity is being preserved in the ongoing ex situ conservation program for this subspecies. A sample of domestic dogs was also included in the analysis for comparison. Seventy-four wolves and 135 dogs were genotyped at 13 unlinked microsatellite loci. The results show that genetic diversity in Iberian wolves is comparable in magnitude to that of other wild populations of gray wolf. Both the wild and the captive Iberian wolf populations have a similarly high genetic diversity indicating that no substantial loss of diversity has occurred in the captive-breeding program. The effective number of founders of the program was estimated as ∼ ∼16, suggesting that all founders in the studbook pedigree were genetically independent. Our results emphasize also the genetic divergence between wolves and domestic dogs and indicate that our set of 13 microsatellite loci provide a powerful diagnostic test to distinguish wolves, dogs and their hybrids.

The Drosophila serido speciation puzzle: putting new pieces together.

The D. seridosuperspecies is a complex mosaic of populations distributed over a vast part of South America and showing various degrees of genetical divergence. We have analyzed its chromosomal constitution in 16 new localities of southeastern and southern Brazil. Both the metaphase and salivary gland chromosomes show a sharp split of these populations in two groups. Four populations, fixed for inversion 2e8and showing the type I karyotype, represent the southwestern limit of D. seridotype B, which inhabits the Cerrado in central-western Brazil. The remaining populations are homozygous for 2x7, an inversion also fixed in the Caatinga populations of northeastern Brazil. However, their karyotype, in those populations analyzed, belong to a different type (V) from that of the Caatinga populations. Populations in this second group are polymorphic for five inversions on chromosome 2 plus another on chromosome 5 and show considerable interpopulation differentiation. The breakpoints of chromosome 2 inversions are described and the inversion loops of several heterokaryotypes are presented. Biogeographical information suggests that there are clear ecological differences between the two groups of populations as well as among the populations within the second group. The possible role of host plants in promoting the genetic divergence among the D. seridopopulations is discussed.

Bottlenecks, population differentiation and apparent selection at microsatellite loci in Australian Drosophila buzzatii.

Species colonizing new areas disjunct from their original habitat may be subject to novel selection pressures, and exhibit adaptive genetic changes. However, if colonization occurs through a small number of founders, the genetic composition of the colonized population may differ from that of the original population simply due to genetic drift. Disentangling the effects of founder drift and selection after colonization is crucial to understanding the adaptive process. Drosophila buzzatii colonized Australia some 600-700 generations ago, and spread rapidly over a wide geographical range. Genetic variation for 15 microsatellite loci in each of nine populations in eastern Australia was used to estimate the size of the bottleneck, and to determine if any of these microsatellites marked genomic regions subject to recent selection. We estimate that on its introduction to Australia, D. buzzatii went through a moderate bottleneck (approximately 30-40 founders). Linkage disequilibrium was common, both intrachromosomal and between loci on different chromosomes. Of the 15 loci, 2 showed evidence of selection, one exhibiting local adaptation in different populations and the other balancing selection. We conclude that linkage disequilibria may be far more common in natural populations than is generally assumed, and the loci apparently affected by selection may well be marking selection in large genome regions including many loci that are not necessarily closely linked.Species colonizing new areas disjunct from their original habitat may be subject to novel selection pressures, and exhibit adaptive genetic changes. However, if colonization occurs through a small number of founders, the genetic composition of the colonized population may differ from that of the original population simply due to genetic drift. Disentangling the effects of founder drift and selection after colonization is crucial to understanding the adaptive process. Drosophila buzzatii colonized Australia some 600–700 generations ago, and spread rapidly over a wide geographical range. Genetic variation for 15 microsatellite loci in each of nine populations in eastern Australia was used to estimate the size of the bottleneck, and to determine if any of these microsatellites marked genomic regions subject to recent selection. We estimate that on its introduction to Australia, D. buzzatii went through a moderate bottleneck (approximately 30–40 founders). Linkage disequilibrium was common, both intrachromosomal and between loci on different chromosomes. Of the 15 loci, 2 showed evidence of selection, one exhibiting local adaptation in different populations and the other balancing selection. We conclude that linkage disequilibria may be far more common in natural populations than is generally assumed, and the loci apparently affected by selection may well be marking selection in large genome regions including many loci that are not necessarily closely linked.