HYBRIDIZATION AS AN EVOLUTIONARY STIMULUS

A hybrid zone between hydrothermal vent mussels (Bivalvia: Mytilidae) from the Mid-Atlantic Ridge

This study provides the first example of a hybrid zone between animal taxa distributed along the mid-ocean ridge system. We examined the distribution and genetic structure of deep-sea hydrothermal vent mussels (Bivalvia: Mytilidae) along a 2888-km portion of the Mid-Atlantic Ridge between 37 degrees 50' N and 14 degrees 45' N latitude. Mitochondrial DNA (mtDNA), allozymes and multivariate-morphometric evidence discriminated between individuals of a northern species, Bathymodiolus azoricus, and a southern species, B. puteoserpentis, that were separated by an intermediate ridge segment almost devoid of mussels. A small sample of mussels from Broken Spur, a vent locality along this intermediate zone, revealed a mixed population with gene frequencies and morphology that were broadly intermediate to those of the northern and southern species. Multilocus clines in mtDNA and allozyme frequencies were centred over the intermediate zone. We consider intrinsic and extrinsic processes that might limit genetic exchange across this hybrid zone.

Genetic composition and diploid hybrid speciation of a high mountain pine, Pinus densata, native to the Tibetan plateau

Pinus densata has been suggested to have originated from hybridization events involving P. tabulaeformis and P. yunnanensis. In this study, allozyme differentiation at 12 loci was studied in 14 populations of P. tabulaeformis, P. densata, and P. yunnanensis from China. The observed genetic composition of P. densata supported the hybrid hypothesis and showed varying degrees of contribution from P. yunnanensis and P. tabulaeformis among its populations. These data, together with previous chloroplast DNA results, indicated different evolutionary histories among P. densata populations. To examine the possibility of ongoing hybridization among the three species, we analyzed patterns of linkage disequilibria between allozyme loci in ovule, pollen, and zygote pools. None of these tests suggested that there is significant ongoing gene exchange, implying that populations of P. densata have a stabilized hybrid nature. The normal fertility and high fecundity of P. densata indicate that this hybrid is maintained through sexual reproduction. P. densata represents an example of diploid hybrid speciation in an extreme ecological habitat that is both spatially and ecologically separated from that of its parents.

Intimately linked or hardly speaking? The relationship between genotype and environmental gradients in a Louisiana Iris hybrid population

Several models of hybrid zone evolution predict the same spatial patterns of genotypic distribution whether or not structuring is due to environment-dependent or -independent selection. In this study, we tested for evidence of environment-dependent selection in an Iris fulva x Iris brevicaulis hybrid population by examining the distribution of genotypes in relation to environmental gradients. We selected 201 Louisiana Iris plants from within a known hybrid population (80 m x 80 m) and placed them in four different genotypic classes (I. fulva, I. fulva-like hybrid, I. brevicaulis-like hybrid and I. brevicaulis) based on seven species-specific random amplified polymorphic DNA (RAPD) markers and two chloroplast DNA haplotypes. Environmental variables were then measured. These variables included percentage cover by tree canopy, elevation from the high water mark, soil pH and percentage soil organic matter. Each variable was sampled for all 201 plants. Canonical discriminant analysis (CDA) was used to infer the environmental factors most strongly associated with the different genotypic groups. Slight differences in elevation (-0.5 m to +0.4 m) were important for distinguishing habitat distributions described by CDA, even though there were no statistical differences between mean elevations alone. I. brevicaulis occurred in a broad range of habitats, while I. fulva had a narrower distribution. Of all the possible combinations, I. fulva-like hybrids and I. brevicaulis-like hybrids occurred in the most distinct habitat types relative to one another. Each hybrid class was not significantly different from its closest parent with regard to habitat occupied, but was statistically unique from its more distant parental species. Within the hybrid genotypes, most, but not all, RAPD loci were individually correlated with environmental variables. This study suggests that, at a very fine spatial scale, environment-dependent selection contributed to the genetic structuring of this hybrid zone.

Evidence for broadscale introgressive hybridization between two redfish (genus Sebastes) in the North-west Atlantic: a rare marine example

The evolutionary importance of introgressive hybridization has long been recognized by plant evolutionists, and there is now a growing recognition for its potential role in animals as well. Detailed empirical investigations of this evolutionary process, however, are still lacking in many animal groups, particularly in the marine environment. Using integrated microsatellite DNA data (eight loci analysed over 803 individuals representing 17 sampling locations) and multivariate statistical procedures (principal component, factorial correspondence and admixture proportion analyses), we: (i) provide a detailed dissection of the dynamics of introgressive hybridization between Sebastes fasciatus and S. mentella, two economically important redfishes from the North-west Atlantic; and (ii) infer the factors potentially involved in the maintenance of the hybrid zone observed in the gulf of St. Lawrence and south of Newfoundland. This study provided one of the rare examples of extensive introgressive hybridization in the ocean, and highlighted the predominant role of this process in shaping the extent of genetic diversity, interspecific differences and population structuring among redfishes from the North-west Atlantic. The extensive (average rate of introgression = 15%) but geographically circumscribed and asymmetrical pattern of introgressive hybridization, the sympatric persistence of two reproductively isolated introgressed groups, the differential patterns of linkage disequilibrium among samples, and the maintenance of genetic integrity of both species outside the defined zone of introgression despite high potential for gene flow, all implicated selection in promoting and maintaining the observed pattern of introgression.

Hybridization as a stimulus for the evolution of invasiveness in plants?

Invasive species are of great interest to evolutionary biologists and ecologists because they represent historical examples of dramatic evolutionary and ecological change. Likewise, they are increasingly important economically and environmentally as pests. Obtaining generalizations about the tiny fraction of immigrant taxa that become successful invaders has been frustrated by two enigmatic phenomena. Many of those species that become successful only do so (i) after an unusually long lag time after initial arrival, and/or (ii) after multiple introductions. We propose an evolutionary mechanism that may account for these observations. Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness. We present and review a remarkable number of cases in which hybridization preceded the emergence of successful invasive populations. Progeny with a history of hybridization may enjoy one or more potential genetic benefits relative to their progenitors. The observed lag times and multiple introductions that seem a prerequisite for certain species to evolve invasiveness may be a correlate of the time necessary for previously isolated populations to come into contact and for hybridization to occur. Our examples demonstrate that invasiveness can evolve. Our model does not represent the only evolutionary pathway to invasiveness, but is clearly an underappreciated mechanism worthy of more consideration in explaining the evolution of invasiveness in plants.

Does interspecific hybridization influence evolutionary rates? An experimental study of laboratory adaptation in hybrids between Drosophila serrata and Drosophila birchii

The low initial fitness of progeny from interspecific crosses in animals and the rarity of interspecific hybridization in natural environments have led to a debate about the evolutionary importance of this phenomenon. Here we directly assess the effects of hybridization between Drosophila serrata and Drosophila birchii on evolutionary rates. We looked at the effects on laboratory adaptation over 30 generations in two laboratory environments, one of which involved nutrition and temperature stress. Laboratory adaptation occurred over time in both environments as reflected by a marked change in viability. However, whilst hybrid lines at no stage performed poorly relative to parental lines, their rate of adaptation never exceeded that of the parentals. Thus, there was no evidence that hybridization increased evolutionary rates. Instead, hybrid lines converged phenotypically with one of the parental species.

Diploid hybrid speciation in Penstemon (Scrophulariaceae)

Hybrid speciation has played a significant role in the evolution of angiosperms at the polyploid level. However, relatively little is known about the importance of hybrid speciation at the diploid level. Two species of Penstemon have been proposed as diploid hybrid derivatives based on morphological data, artificial crossing studies, and pollinator behavior observations: Penstemon spectabilis (derived from hybridization between Penstemon centranthifolius and Penstemon grinnellii) and Penstemon clevelandii (derived from hybridization between P. centranthifolius and P. spectabilis). Previous studies were inconclusive regarding the purported hybrid nature of these species because of a lack of molecular markers sufficient to differentiate the parental taxa in the hybrid complex. We developed hypervariable nuclear markers using inter-simple sequence repeat banding patterns to test these classic hypotheses of diploid hybrid speciation in Penstemon. Each species in the hybrid complex was genetically distinct, separated by 10-42 species-specific inter-simple sequence repeat markers. Our data do not support the hybrid origin of P. spectabilis but clearly support the diploid hybrid origin of P. clevelandii. Our results further suggest that the primary reason diploid hybrid speciation is so difficult to detect is the lack of molecular markers able to differentiate parental taxa from one another, particularly with recently diverged species.

Genetics and the origin of bird species

External (environmental) factors affecting the speciation of birds are better known than the internal (genetic) factors. The opposite is true for several groups of invertebrates, Drosophila being the outstanding example. Ideas about the genetics of speciation in general trace back to Dobzhansky who worked with Drosophila. These ideas are an insufficient guide for reconstructing speciation in birds for two main reasons. First, speciation in birds proceeds with the evolution of behavioral barriers to interbreeding; postmating isolation usually evolves much later, perhaps after gene exchange has all but ceased. As a consequence of the slow evolution of postmating isolating factors the scope for reinforcement of premating isolation is small, whereas the opportunity for introgressive hybridization to influence the evolution of diverging species is large. Second, premating isolation may arise from nongenetic, cultural causes; isolation may be affected partly by song, a trait that is culturally inherited through an imprinting-like process in many, but not all, groups of birds. Thus the genetic basis to the origin of bird species is to be sought in the inheritance of adult traits that are subject to natural and sexual selection. Some of the factors involved in premating isolation (plumage, morphology, and behavior) are under single-gene control, most are under polygenic control. The genetic basis of the origin of postmating isolating factors affecting the early development of embryos (viability) and reproductive physiology (sterility) is almost completely unknown. Bird speciation is facilitated by small population size, involves few genetic changes, and occurs relatively rapidly.

Hybridization of Bird Species

Hybridization, the interbreeding of species, provides favorable conditions for major and rapid evolution to occur. In birds it is widespread. Approximately one in ten species is known to hybridize, and the true global incidence is likely to be much higher. A longitudinal study of Darwin's finch populations on a Galápagos island shows that hybrids exhibit higher fitness than the parental species over several years. Hybrids may be at an occasional disadvantage for ecological rather than genetic reasons in this climatically fluctuating environment. Hybridization presents challenges to the reconstruction of phylogenies, formulation of biological species concepts and definitions, and the practice of biological conservation.

Evolutionary analysis of Pinus densata Masters, a putative Tertiary hybrid : 1. Allozyme variation

Allozyme differentiation at 13 loci was studied in populations of Pinus tabulaeformis, P. densata, and P. yunnanensis from China. It was previously suggested that P. densata represents a Tertiary hybrid between P. tabulaeformis and P. yunnanensis. The observed levels of allozyme variation within and among the investigated species were comparable to those of other conifers. P. tabulaeformis differed markedly from P. yunnanensis with respect to allozyme frequencies, while P. densata was intermediate between the two putative parents. There was evidence of homozygote excess in embryos from all investigated species, as compared to Hardy-Weinberg expectations. The observed allozyme composition of P. densata conformed to earlier morphological and molecular evidence indicating hybrid origin of this taxon. It was proposed that fusion of gene pools from P. tabulaeformis and P. yunnanensis has led to adaptive evolution of a new species, P. densata.

Evolutionary analysis of Pinus densata (Masters), a putative Tertiary hybrid. : 2. A study using species-specific chloroplast DNA markers

Restriction fragment analysis and heterologous hybridization of chloroplast (cp) DNA was used to develop species-specific markers for P. tabulaeformis, P. yunnanensis and P. massoniana. Fragment patterns created by the BclI and DraI restriction enzymes and hybridization patterns to the psbC and psbD probes were distinctive among the three species. No intraspecific variation was detected with respect to any of the cpDNA markers developed in this study. The cpDNA markers obtained were subsequently used to examine the parentage of P. densata, a putative Tertiary hybrid between P. tabulaeformis and P. yunnanensis. The analysis demonstrated for the first time that P. densata populations accommodate chloroplast genomes of P. tabulaeformis and P. yunnanensis, which strongly supports earlier suggestions of the hybrid origin of this species. It appears that P. densata represents a stabilized natural hybrid that has become adapted to high mountain environments where neither of the parental species can normally grow.

Beginnings of Fruit Growing in the Old World

The article reviews the available information on the start of fruit tree cultivation in the Old World. On the basis of (i) evaluation of the available archeological remains and (ii) examination of the wild relatives of the cultivated crops, it was concluded that olive, grape, date, and fig were the first important horticultural additions to the Mediterranean grain agriculture. They were most likely domesticated in the Near East in protohistoric time (fourth and third millennia B.C.) and they emerge as important food elements in the early Bronze Age. Domestication of all four fruit trees was based on a shift from sexual reproduction (in the wild) to vegetative propagation of clones (under domestication). Olive, grape, date, and fig can be vegetatively propagated by simple techniques (cuttings, basal knobs, suckers) and were thus preadapted for domestication early in the development of agriculture. The shift to clonal propagation placed serious limitations on selection and on fruit set under cultivation. We have examined the consequences of this shift in terms of the genetic makeup of the cultivars and traced the various countermeasures that evolved to ensure fruit set. Finally, it was pointed out that in each of these classic fruit trees we are confronted with a variable complex of genuinely wild types, secondary weedy derivatives and feral plants, and groups of the domesticated clones, which are all interfertile and interconnected by occasional hybridization. It was concluded that introgression from the diversified wild gene pool facilitated the rapid buildup of variation in the domesticated crops.

Studies on some genetic parameters of rice (Oryza sativa L.)

In order to utilize the available useful variation in breeding rice to improve yield and quality of grain, the performance of a wide range of rice germ plasm comprising 30 varieties was evaluated in northern India, a major rice growing belt. Plant performance revealed high genetic divergence and phenotypic variability in the crop, with the maximum range of variation being for grain number per panicle and the minimum for grain dimensions. There were also significant differences among varietal means for ten phenotypic traits. Genotypic and phenotypic variance contributed profoundly to the variance of the phenotypic traits studied, but, since genetic variability in the traits related to yield was considerable, there is scope for further improvement in yielding ability. Grain number per panicle, number of effective tillers per plant and culm length exhibit high heritability, and genotypic coefficient of variation and therefore a high genetic advance. Thus, selection for these traits would be effective in crop improvement. Moreover, grain number per panicle shows a significant positive correlation with yield, and this trait could profitably form a reliable index for the yielding capacity of this crop.