Pure Parental Species Research Articles

Distribution and habitat use of water frog hybrid complexes in France

AbstractHybrid zones are either distributed along clines or in a mosaic of patches. This distribution may depend upon variation in taxon habitat use. Habitat use and distribution of diverse taxa of water frogs (Rana ridibunda,R. lessonae,R. perezi,R.kl.grafiandR.kl.esculenta) in France are analysed to determine whether water frog complexes conform to the mosaic or clinal model. Biogeographical scenarios may be invoked in order to explain the distribution of water frogs. However, the distribution ofR. pereziandR.kl.grafi, being restricted to regions characterized by Mediterranean or Oceanic climatic conditions, suggests that these frogs do not endure cold winters.R. ridibundais widespread in Southern France and its distribution suggests multiple introductions. It is concluded that water frogs conform to the mosaic zone model rather than to the tension zone model because: (i) taxa exhibited differences in habitat use, (ii) pure parental species were documented and (iii) hybrids are not unfit relative to parental species.

Reproductive Success and Assortative Mating amongPomoxisSpecies and Their Hybrids

We investigated the patterns of reproductive isolation and gene flow between crappie species Pomoxis spp. by conducting a series of controlled breeding experiments using both pure parental species and various forms of interspecific hybrids (i.e., F1, F2) as parents. To assess relative reproductive success of mating types, progeny from each of three different experimental treatments were assayed for genotypic composition at four diagnostic allozyme loci. In the first experimental treatment, we confirmed the viability and fertility of F1 hybrids and observed that genotypic and allelic frequencies among the F2 progeny departed from predicted Mendelian genotypic proportions. In the second experimental treatment, we confirmed that F3 hybrids can arise from intermating F2 hybrids. However, the small number of progeny recovered from experimental ponds suggests that either prezygotic or postzygotic barriers to gene flow had limited reproductive success. In the third experimental treatment, we evaluated the relative reproductive success of the parental species and both reciprocal F1 hybrids. We rejected the null hypothesis of random mating from equal mixtures of parental and hybrid brood based on a maximum likelihood mixture model estimate of resulting progeny. Within each replicate, one or both parental species were most successful, although F2 hybrid progeny also occurred more often than expected; first generation hybrids and backcrosses occurred less frequently than expected. These and previously reported observations concerning the viability of hybrids and backcrosses indicate that the two crappie species can interbreed but that they mate assortatively. Because reproductive isolation is imperfect, we caution against intentional mixing or creation of zones of secondary contact between these species.

Effects of host plant hybridization on resistance to willow leaf rust caused by Melampsora sp.

SummaryPatterns of resistance to willow leaf rust caused by Melampsora sp. were measured on Salix eriocephala, Salix sericea, and their interspecific hybrids in natural field populations in 1991–94 and on potted plants in common garden experiments in 1994 and 1995. Testing was carried out to establish if the resistance of hybrids was intermediate between pure parental species (additive), equal to one of the parents (dominant), lower than both parents (susceptible), higher than both parents (resistant), or not different than either parent (null hypothesis). Support was generally found for hybrid susceptibility in the field and common garden experiments, with the null hypothesis of no difference having some support. The results from the common garden study suggest that the susceptibility of hybrids seen in the field is due to genetic differences. Hybridization between these species of willows results in increased susceptibility to this pathogen. The variation among years in the relative susceptibility of hybrid and parental willows may be due to environmental effects on the willows or to shifts in the abundance of Melampsora races.

Fluctuating asymmetry in Atlantic salmon, European trout and their hybrids, including triploids

Developmental stability as measured by fluctuating asymmetry is reported to be greater in interspecific salmonid hybrids than in the pure parental species. We have examined FA in Atlantic salmon, Salmo salar and European trout, Salmo trutta and in hybrids between them. FA is significantly greater in meristic traits (number of gill rakers, pectoral fin rays and pelvic fin rays) in hybrids but not in morphometric traits (length of maxilla, length of head, opercular length and eye diameter) in which the hybrids show intermediate values. Triploidisation of salmon had no statistically significant effect on meristic FA. Triploidisation of hybrids significantly reduced meristic FA to values close to those of diploid salmon, thereby reversing the hybridization effect. Triploidisation had little effect on morphometric FA in any of the samples. We conclude that FA in meristic and morphometric traits must be considered separately, and the increased meristic FA in hybrids represents a break-down of coadaptation as mediated by relational genic balance between chromosomes or chromosomal segments.

Morphometric and meristic characters in salmon, Salmo salar L., trout, Salmo trutta L., and their hybrids

Abstract. Atlantic salmon, Salmo salar L., brown trout, S. trutta L. and their hybrids are normally identified in the field by empirical differences in maxilla length, thickness of the caudal peduncle, degree of forking of the tail and overall body conformation. This study quantifies these characters and analyses their variations in electrophoretically identified hatchery-reared individuals. Means and variances of the various measures are presented for salmon, trout and hybrids separately. The morphometric characters do not satisfactorily distinguish hybrids from the pure species, and even within samples of the pure species, some individuals will be misclassified as hybrids, or as members of the opposite species, if single characters are used on their own. Hybrids often resemble one or other parent species in one or more characters and are less often intermediate in phenotype. Triploidized hybrids are more like salmon than diploid hybrids are, and triploidized salmon are not different from diploid salmon. The results confirm that frequencies of hybrids of these species cannot be reliably assessed by morphological characters alone, and even for individuals of the pure parental species, independent confirmation of species status is advisable. Early reports of hybrid frequencies in wild stocks should be treated with caution, and apparently higher levels of hybridization in more recent studies compared with older, traditional surveys may simply reflect the greater precision of electrophoretic identification.

Genome exclusion and gametic DAPI-DNA content in the hybridogenetic Bacillus rossius-grandii benazzii complex (Insecta Phasmatodea).

Among Sicilian stick insects, two hybridogenetic complexes have been discovered: Bacillus rossius-grandii benazzii and B. rossius-grandii grandii, which also produce androgenetic offspring. The egg maturation of the former is analyzed here through DAPI fluorometry, which, besides the assessment of the meiotic stages, also allows their DNA measurements and the analysis of sperm-head evolution into male pronuclei in these polyspermic eggs. Hybridogenetic eggs undergo an extrasynthesis of chromosomes, because two groups of n autobivalents (4C each) are segregated at metaphase 1st; the two groups must correspond to the pure parental species haplosets. Then the grandii chromosomes degenerate (1st polar body), while the rossius chromosomes divide further to produce two groups of n autodiads (2C each); one of them degenerates (2nd polar body), and the other is ready to perform syngamy (female pronucleus). Meanwhile, several B. grandii sperm evolve into male pronuclei by doubling their DNA (from 1C to 2C content) and assuming an interphase nucleus appearance. If regular mixis occurs, the F1 hybrid constitution is restored but, if it fails, a fusion between two sperms may occur, originating fully paternal descendants (natural androgenesis). The genome exclusion mechanism of stick-insect hybridogens appears to be more primitive than those observed in the already known hybridogenetic complexes of Poeciliopsis and Rana esculenta. Unfertilized eggs of hybridogens are capable of self-activation, but the cytology of the related clonally reproducing B. whitei indicates that its parthenogenetic mechanism stems from the hybridization event (hybrid theory) rather than from tychoparthenogenetic potentialities (spontaneous theory).

The effects of assortative mating and migration on cytonuclear associations in hybrid zones.

We examine the influence of nonrandom mating and immigration on the evolutionary dynamics of cytonuclear associations in hybrid zones. Recursion equations for allelic and genotypic cytonuclear disequilibria were generated under models of (1) migration alone, assuming hybrid zone matings are random with respect to cytonuclear genotype; and (2) migration in conjunction with refined epistatic mating, in which females of the pure parental species preferentially mate with conspecific males. Major results are as follows: (a) even the slightest migration removes the dependency of the final outcome on initial conditions, producing a unique equilibrium in which both pure parental genotypes are maintained in the hybrid zone; (b) in contrast to nuclear genes, the dynamics of cytoplasmic allele frequencies appear robust to changes in the assumed mating system, yet are particularly sensitive to gene flow; (c) continued immigration can generate permanent cytonuclear disequilibria, whether mating is random or assortative; and (d) the order of population censusing (before versus after reproduction by immigrants) can have a dramatic effect on the magnitude but not the pattern of cytonuclear disequilibria. Using the maximum likelihood method, the parameter space of migration rates and assortative mating rates was examined for best fit to observed cytonuclear disequilibria data in a hybrid population of Hyla tree frogs. An epistatic mating model with a total immigration rate of about 32% per generation produces equilibrium gene frequencies and cytonuclear disequilibria consistent with the empirical observations.

A Specific and Predictable Biochemical Anomaly in Interspecific Hybrids of Baptisia viridis × B. leucantha

IN a chromatographic analysis of twenty-one putative hybrids of Baptisia viridis × B. leucantha each of the hybrid types contained in the leaves 4 distinctive substances which were absent from the leaves of more than 25 specimens of each of the pure parental species examined. The 4 hybrid-specific components were distributed in a novel pattern and reacted characteristically in ammonia vapour when observed in ultra-violet light. Accordingly, they were considered to be bona fide hybrid-specific substances1. Since there has been considerable interest in the appearance of new components in hybrids, and attendant theoretical implications touch on aspects of plant biochemistry and evolutionary mechanisms, it is important to establish conclusively that such substances actually are hybrid-specific and, if possible, their identity.