Evidence Of Backcrossing Research Articles

First evidence for backcrossing of F1 hybrids in Acropora corals under sperm competition

Acropora is a species-rich genus of reef-building corals with highly diverse morphologies. Hybridization among intercrossing species potentially influences species diversity within Acropora. However, the mechanisms that allow hybridization/backcrossing remain unknown. Although we tested a limited number of species, we hypothesized that Acropora gametes in the Indo-Pacific may preferentially fertilize conspecific gametes despite their compatibility with heterospecific gametes, leading to infrequent hybridization between potentially intercrossing species. In this study, F1 hybrids of Acropora florida and A. intermedia showed specific fertilization trends. For example, sperm had the ability to backcross with the parental species even in the presence of sperm from the parental species. Also, eggs of the hybrids produced from A. florida eggs and A. intermedia sperm (“FLOint”) exhibited self-fertilization. Since a low ratio of hybridization between A. florida and A. intermedia is predicted, the population size of hybrids should be small. Therefore, self-fertilization would facilitate reproduction of the hybrid in nature, while remaining sperm could outcompete parental species sperm to backcross with eggs. Although we succeeded in breeding two colonies of hybrids, it is reasonable to speculate that hybrids show a high tendency to choose the most efficient sexual reproduction tactics.

Cryptic introgression into the kidney saxifrage (Saxifraga hirsuta) from its more abundant sympatric congener Saxifraga spathularis, and the potential risk of genetic assimilation.

Although hybridization can play a positive role in plant evolution, it has been shown that excessive unidirectional hybridization can result in replacement of a species' gene pool, and even the extinction of rare species via genetic assimilation. This study examines levels of introgression between the common Saxifraga spathularis and its rarer congener S. hirsuta, which have been observed to hybridize in the wild where they occur sympatrically. Seven species-specific single nucleotide polymorphisms (SNPs) were analysed in 1025 plants representing both species and their hybrid, S. × polita, from 29 sites across their ranges in Ireland. In addition, species distribution modelling was carried out to determine whether the relative abundance of the two parental species is likely to change under future climate scenarios. Saxifraga spathularis individuals tended to be genetically pure, exhibiting little or no introgression from S. hirsuta, but significant levels of introgression of S. spathularis alleles into S. hirsuta were observed, indicating that populations exhibiting S. hirsuta morphology are more like a hybrid swarm, consisting of backcrosses and F2s. Populations of the hybrid, S. × polita, were generally comprised of F1s or F2s, with some evidence of backcrossing. Species distribution modelling under projected future climate scenarios indicated an increase in suitable habitats for both parental species. Levels of introgression observed in this study in both S. spathularis and S. hirsuta would appear to be correlated with the relative abundance of the species. Significant introgression of S. spathularis alleles was detected in the majority of the S. hirsuta populations analysed and, consequently, ongoing introgression would appear to represent a threat to the genetic integrity of S. hirsuta, particularly in areas where the species exists sympatrically with its congener and where it is greatly outnumbered.

Hybridization but No Evidence for Backcrossing and Introgression in a Sympatric Population of Great Reed Warblers and Clamorous Reed Warblers

Hybridization is observed frequently in birds, but often it is not known whether the hybrids are fertile and if backcrossing occurs. The breeding ranges of the great reed warbler (Acrocephalus arundinaceus) and the clamorous reed warbler (A. stentoreus) overlap in southern Kazakhstan and a previous study has documented hybridization in a sympatric population. In the present study, we first present a large set of novel microsatellite loci isolated and characterised in great reed warblers. Secondly, we evaluate whether hybridization in the sympatric breeding population has been followed by backcrossing and introgression.We isolated 181 unique microsatellite loci in great reed warblers. Of 41 loci evaluated, 40 amplified and 30 were polymorphic. Bayesian clustering analyses based on genotype data from 23 autosomal loci recognised two well-defined genetic clusters corresponding to the two species. Individuals clustered to a very high extent to either of these clusters (admixture proportions ≥0.984) with the exception of four previously suggested arundinaceus–stentoreus hybrid birds that showed mixed ancestry (admixture proportions 0.495–0.619). Analyses of simulated hybrids and backcrossed individuals showed that the sampled birds do not correspond to first–fourth-generation backcrosses, and that fifth or higher generation backcrosses to a high extent resemble ‘pure’ birds at this set of markers.We conclude that these novel microsatellite loci provide a useful molecular resource for Acrocephalus warblers. The time to reach reproductive isolation is believed to be very long in birds, approximately 5 Myrs, and with an estimated divergence time of 2 Myrs between these warblers, some backcrossing and introgression could have been expected. However, there was no evidence for backcrossing and introgression suggesting that hybrids are either infertile or their progeny inviable. Very low levels of introgression cannot be excluded, which still may be an important factor as a source of new genetic variation.

Climate change induced hybridization in flying squirrels

AbstractThere is now unequivocal evidence for global climate change; however, its potential impacts on evolutionary processes remain unclear. Many species have responded to contemporary climate change through shifts in their geographic range. This could lead to increased sympatry between recently diverged species; likely increasing the potential for hybridization. Recently, following a series of warm winters, southern flying squirrels (Glaucomys volans) in Ontario, Canada rapidly expanded their northern range limit resulting in increased sympatry with the closely related northern flying squirrel (Glaucomys sabrinus). This provided the opportunity to test the prediction that contemporary climate change can act as a catalyst creating conditions for the formation of hybrid zones. Following extensive sampling and molecular analyses (nuclear and mitochondrial DNA), we identified the occurrence of hybridization between sympatricG. sabrinusandG. volans. There was evidence of backcrossing but not of extensive introgession, consistent with the hypothesis of recent rather than historic hybridization. To our knowledge, this is the first report of hybrid zone formation following a range expansion induced by contemporary climate change. This is also the first report of hybridization between North American flying squirrel species.

Genetic analysis of a wild-caught hybrid between non-sister Heliconius butterfly species.

Interspecific hybridization occurs regularly in wild Heliconius butterflies, although hybrid individuals are usually very rare. However, hybridization generally occurs only between the most closely related species. We report a rare naturally occurring hybrid between non-sister species and carry out the first genetic analysis of such distant hybridization. Mitochondrial and nuclear genes indicate that the specimen is an F1 hybrid between a female Heliconius ethilla and a male Heliconius melpomene, originating from a group of 13 species estimated to have diverged over 2.5 Myr ago. The presence of such distant natural hybrids, together with evidence for backcrossing, suggests that gene flow across species boundaries can take place long after speciation. Adaptive genes such as those involved in wing coloration could thus be widely shared among members of this highly mimetic genus.

A secondary hybrid zone between diploid Dactylorhiza incarnata ssp. cruenta and allotetraploid D. lapponica (Orchidaceae)

Secondary hybrid zones are not uncommon in Dactylorhiza, but knowledge of ecological and evolutionary consequences of hybridization are scarce. Here, we assess interploidal gene flow and introgression in a hybrid zone between diploid Dactylorhiza incarnata ssp. cruenta (2n = 2x = 40) and its putative allotetraploid derivative D. lapponica (2n = 4x = 80). Photometric quantification of DNA content and morphology confirmed that triploids are abundant in sympatric populations in our study area. Allozyme segregation patterns in D. lapponica supported an allopolyploid origin, although unbalanced genotypes suggested rare pairings between homoeologous chromosomes. Photometric data and chromosome counts suggest backcrossing between the triploid hybrid and D. lapponica, and hence some hybrid fertility. Triploids are morphologically more similar to the tetraploids than the diploids, maybe owing to the hybrid origin of both triploids and tetraploids. The diploids and tetraploids were not more similar in the parapatric populations compared to when they occur in allopatry. This indicates that backcrossing rarely leads to introgression, or alternatively that allopatric populations are not isolated enough to prevent influx of pollen from the other species. Despite some evidence of backcrossing, our study gives few indications that widespread hybridization entails local breakdown of species boundaries. Rather, the hybrid zone may be a transient phenomenon due to intensive mowing, resulting in the opening of habitats and hence bringing the parental species into close contact.

Genetic evidence of natural hybridization between Podarcis sicula and Podarcis tiliguerta (Reptilia: Lacertidae)

AbstractAn allozyme survey revealed instances of natural hybridization between Podarcis sicula and P. tiliguerta from a locality of southern Sardinia. Pure specimens of both species together with some F1 hybrids were found, but no evidence of backcrossing was detected. The hybrid specimens appeared morphologically intermediate between the parent species. The absence of backcrossed individuals indicate that free interbreeding between the two species is unlikely. Hybrids were found in areas of habitat disturbance only.

Natural Hybridization between Diploid Crucian Carp Species and Genetic Independence of Triploid Crucian Carp Elucidated by DNA Markers

The population structure of genus Carassius in Lake Koyama, southeast Japan, was analyzed by genetic markers as microsatellite DNA, mtDNA RFLP and isozymes. Based on the ploidy level and morphological analysis, four Carassius groups were detected. The triploid group was identified as Ginbuna (C. langsdorfii). In the diploid group, Nagabuna (C. burugeri sp) and Gengoroubuna, (C. cuvieri) were identified. Remaining diploid individuals had morphological traits that were intermediate between Nagabuna and Gengoroubuna. These were considered as hybrids and their descendants. From the results of mtDNA RFLP and isozyme patterns, the triploid population was considered to be independent from the gene pools of diploid. The hybrids had the mtDNA haplotypes which were common to Gengoroubuna and Nagabuna populations. Based on the three microsatellite loci, Ginbuna was classified into six clonal lines. In the diploid population, substitution of the major alleles of Nagabuna and Gengoroubuna were found. The hybrids had alleles that were common in Nagabuna and Gengoroubuna. The values of the hybrid index (IH) which are ranged from 0.771 to 0.964 in Nagabuna, from 0.102 to 0.806 in the hybrids and from 0.068 to 0.157 in Gengoroubuna. The hybrid population was verified to be derived from crossbreeding between the Gengoroubuna and Nagabuna populations. Evidence of backcrossing in nature by microsatellite DNA markers was also obtained in the diploid populations.

Electrophoretic evidence for hybridization in the ant genus Acanthomyops (Hymenoptera: Formicidae)

Abstract Enzyme electrophoresis on a GPI locus was used to provide the first genetic tests of the hypotheses that hybridization occurs between the ant species Acanthomyops latipes and A . claviger , that the hybrids are F 1 ’s, and that the hybrids result from a latipes queen mating with a claviger male. A difference in GPI allele frequencies (fixed in latipes , polymorphic but strongly skewed to an alternate allele in claviger ) was used to test five predictions that arise from these hypotheses. Test results were consistent with all components of these hypotheses, with the possible exception of a reciprocal cross in one colony. No evidence of backcrossing was found. We believe that two alternative hypotheses to explain the existence of morphologically intermediate queens can be rejected: the hypothesis that latipes produces dimorphic queens, and the hypothesis that colonies producing the morphologically intermediate queens represent a separate species. The pattern of hybridization, with F 1 hybrids commonly occurring throughout the broad range of sympatry of the parent species, contrasts sharply with most “hybrid zone” phenomena.

Confirmation of the Hybrid Origin of Chiloglottis × pescottiana (Orchidaceae: Diurideae). I. Genetic and Morphometric Evidence

Pollination by sexual deception in orchids is characterised by a high degree of pollinator specificity, which may account for the rarity of natural hybrids within the group. Only one such hybrid has been formally recognised in Australia, Chiloglottis × pescottiana R.S.Rogers, which has intermediate floral morphology between its putative parents, C. valida D.L.Jones and C. trapeziformis Fitzg. In this paper, genetic and morphometric analyses confirm the hybrid origin of this taxon. Allozyme analysis of C. trapeziformis and C. valida revealed fixed allelic differences at four ‘diagnostic’ loci and significant frequency differences at three other loci. In all cases, C. × pescottiana exhibited fixed heterozygosity at the diagnostic loci. Multidimensional scaling of both the genetic data and seven morphometric traits revealed distinct clusters of C. trapeziformis and C. valida while C. × pescottiana formed an intermediate cluster between the two parents. To test for genetic compatibility between C. trapeziformis, C. valida and C. × pescottiana, a series of reciprocal artificial crosses were performed. In all cases, the percentage of capsules formed was at least as great for between-species crosses as for within-species selfs and crosses (range 75–100%). No significant differences in the percentage of seed with normal embryos was detected between self- and cross-pollinations within C. trapeziformis (range 77–81%), C. valida (range 59–74%) and C. × pescottiana (range 30–51%), but the percentage of normal embryos was notably lower in C. × pescottiana. The cross C. trapeziformis female by C. valida male produced significantly more normal embryos (90%) compared with the reverse cross (46%). Artificial backcrosses of C. × pescottiana to C. valida and C. trapeziformis had lower percentages of normal embryos when C. × pescottiana was the pollen donor (39–43%) rather than recipient (62–68%), suggesting reduced pollen viability in the latter taxon. The size of F2 embryos in C. × pescottiana seed capsules was smaller than the embryos of both C. valida and C. trapeziformis. Despite confirmation of hybridisation, little evidence for backcrossing was found. Thus, while the specific pollinator relationships may occasionally break down in these sexually deceptive orchids, reduced viability of hybrid pollen and F2 seed, and inefficient pollination of the hybrid, may minimise introgression. It is concluded from the available evidence that hybridisation has not been a major evolutionary factor in the diversification of sexual deception worldwide.

Temporal changes in the genetic structure of the Daphnia species complex in Tjeukemeer, with evidence for backcrossing

Temporal genetic variation was studied within the cyclic parthenogenetic Daphnia galeata-cucullata species complex in Lake Tjeukemeer (The Netherlands). During three successive years, three allozyme loci were studied in order to compare the level of genetic variation in D. galeata (G), D. cucullata (C) and their interspecific hybrid (C × G). This data set was used to investigate: whether C × G hybrids are regularly produced by the parental species and to what extent backcrossing takes place in this species complex. G and C × G were the most abundant taxa, whereas C was only abundant in late summer and autumn. Genetic diversity was highest in C, lowest in G, and intermediate in C × G. pgm and pgi allele frequencies of C × G were not intermediate to those of the parental species but were more similar to G than to C. In addition, almost no significant deviations from Hardy-Weinberg were observed. An UPGMA analysis of Nei's genetic distances showed that C × G hybrids are more closely related to G (difference <0.04), than to C (difference 0.30). My data suggest, in accordance with other studies, that C × G hybrids are probably still formed by the parental species, though not necessarily every year. Furthermore, my data suggest that backcrossing occurred frequently, and led to introgression of C alleles into G.

Hybridization between tetraploid species of Alopecurus L. (Poaceae): morphological studies of natural and artificial hybrids

The morphology of individuals of mixed natural populations of three species of Alopecurus, A. pratensis, A. geniculars and A. arundinaceus, is described. It is suggested that plants morphologically intermediate between the parental species are hybrids or of hybrid origin. Hybridization occurs more readily between A. pratensis and A. arundinaceus than between A. pratensis and A. geniculate, since intermediate forms were recovered from all mixed populations of A. pratensis and A. arundinaceus. The intermediate forms occur only in a few populations of A. pratensis and A. geniculars where an appropriate habitat is available. Evidence of backcrossing and therefore introgression comes from the continuum of intermediate forms that exists between the species. Scatter diagrams for individual populations show distinct differences in the degree of introgression. Observations on artificially produced hybrids are similar to those on naturally occurring hybrids.

An Analysis of Natural Hybridization between the Pocket gophers, Thomomys bottae and Thomomys umbrinus, in Arizona

Natural hybridization between Thomomys bottae and T. umbrinus at a contact locality in the Patagonia Mountains of southern Arizona is documented utilizing chromosomal and exomorphological characteristics. The hybridization is unbalanced in favor of F1 production with little evidence of backcrossing. Moreover, there is no indication of genic introgression between the two species. A post-mating reproductive barrier due to meiotic imbalance results in nearly complete sterility in male hybrids and a significantly lowered reproductive potential in female hybrids. Partial development of a pre-mating barrier based on differential parental habitat selection and behavioral differences is suggested, and reasons why such secondary barriers have not developed fully are discussed. Ecological relationships between T. bottae and T. umbrinus at this locality suggest that the niche of the former species is best considered a subset of the latter.

Morphological and Myogen Comparison of Johnny and Tessellated Darters and Their Hybrids, Genus Etheostoma, Near Ottawa, Canada

Ten morphological characters and two electrophoretic characters were studied in nearly 200 specimens of johnny and tessellated darters from the Ottawa area. The data were analyzed using a simple character index and principal component analysis. The frequency distribution of the character index was trimodal, and specimens could be resolved into three groups: those typical of johnny and tessellated darters, and a group of intermediates. The two extreme groups were interpreted as representing two valid species, Etheostoma nigrum Rafinesque and Etheostoma olmstedi Storer, and part of the intermediate group was inferred to consist of interspecific hybrids. Hybrids were estimated to constitute less than 7.6%. There was no clear evidence of backcrossing. Most specimens could be clearly identified with one or the other of the two species. A key is provided.

Hybridization and Isolating Mechanisms Between Catostomus commersonii and C. macrocheilus (Pisces: Catostomidae)

Catostomus commersonii and C. macrocheilus are sympatric in three major drainages of British Columbia. Morphological analysis employing a character index and discriminant function analysis of allopatric and sympatric populations revealed that hybrids were in 9 of the 11 lakes examined in which both species occurred. Presumed F1 hybrids made up an average of 7% of the individuals of the combined populations where hybrids occurred. There was no evidence of backcrossing or swamping.Segregation in spawning time and habitat was not apparent in four confined inlets which have migrations of both species. Some lakes supporting both species were found to have inlets in which only one species spawned. Temporal isolation was absent but habitat isolation was present in some areas of Cluculz River, an outlet with diversified habitat. Where segregation was apparent, C. commersonii spawned in shallow gravelly areas and C. macrocheilus in deep sandy areas. Artificial crosses of the species, two backcrosses involving hybrid females, and microscopic examination of hybrid testes failed to show any effective postmating isolating mechanism. An interaction of temporal, habitat, and postmating isolating mechanisms was not considered sufficient to prevent swamping. Ethological isolation is inferred to be of major importance in reducing initial hybridization. No explanation was found for the apparent failure of hybrids to successfully reproduce.Hybridization could not be attributed to environmental factors commonly associated with this phenomenon. Males of one species entering the spawning act with males and a female of the other species is considered the main cause of hybridization. Mis-mated individuals, however, have much opportunity for conspecific matings at other times. There was no evidence of reinforcement of isolating mechanisms despite the probable occurrence of hybridization in some areas for at least several hundred years. Characters of each species were as distinctive in the areas of sympatry as in the areas of allopatry. Natural selection against individuals producing hybrids is apparently slow.