How does hybridization provide genetic variation

F 1 versus backcrosses; Fig. The PCoA analysis suggested an introgression pattern toward U. Manual classification yielded slightly different hybrid assignments than did the program NewHybrids, which identified 34 F 1 individuals as opposed to 32 F 1 individuals determined manually , six as BC 1 instead of 11 manually and 11 BC 2 instead of 17 manually Fig. Principal coordinates analyses P.

In the NewHybrids plots, each individual is represented by a thin vertical line divided into eight colored segments that represent the individual's estimated membership fractions to each of the eight cross types.

Clearly, hybridization increased the level of genetic diversity in present-day US populations because alleles were identified in the 60 hybrid individuals 39 U. The alleles not previously identified in either species may result from limited sampling of parental species or represent alleles that originated in the sampled populations via mutation. The level of genetic diversity in the naturalized U.

Excluding hybrids, we observed between three and four alleles per locus per population and moderate levels of observed heterozygosity range of 0. Little inbreeding was detected in adult trees in these naturalized populations and the slightly negative inbreeding coefficients in several of these populations suggest an excess of heterozygotes in these populations F -values; Table 4. The presence of hybrids clearly increased the genetic diversity of naturalized populations, and the increase in the number of alleles and effective alleles per population was related to the proportion of hybrids detected in a population Table 4 ; Fig.

The presence of hybrids also increased the levels of heterozygosity and reduced estimates of the inbreeding coefficient more negative F -values; Table 4. Such trends are expected given that many of the trees in these naturalized populations are F 1 hybrids and are thus heterozygous at all species-specific loci.

Genetic diversity characteristics of Ulmus pumila populations not bold: including hybrids; bold: excluding hybrids based on 13 microsatellite loci. Multilocus averages : N a , observed number of alleles; N a Freq. In order to compare the genetic diversity that may have existed at the time U. Hybrids increased the genetic diversity of the US populations and rendered the US populations slightly more diverse relative to the East Asian accessions.

Ulmus pumila from both East Asia and the USA appear genetically variable and heterozygous with little evidence of inbreeding Table 4. Analysis of molecular variance for eight naturalized Ulmus pumila populations estimates not bold and for a subset of these samples determined to be genotypically pure individuals within each population in bold.

The overall F ST -value for the eight naturalized populations was 0. There was little genetic differentiation between the U. The greatest levels of genetic differentiation were found among some of the naturalized populations, although only moderate levels of genetic differentiation were observed pairwise F ST -range 0.

When hybrids were included in the analyses, the level of genetic differentiation increased for 13 of the pairwise cases, decreased for eight cases and remained unchanged for the other seven Table 6.

Pairwise genetic differentiation F ST among Ulmus pumila naturalized populations including hybrids upper diagonal and excluding hybrids below diagonal. The Mean F ST -value excludes hybrids. The non-mean F ST -values in bold are significantly different from 0. When we contrasted only the eight naturalized US U. Significant admixture levels were observed between populations as each population and individuals within each population were comprised of these four genetic clusters. In the structure plots each individual is represented by a thin vertical line divided into K -colored segments that represent the individual's estimated membership fractions in each of the K clusters.

Black lines separate populations labeled at the bottom. Our sampling strategy in the USA targeted morphologically typical U. Like many successful invasive woody plants, U. Our data indicate widespread hybridization between U. It also confirms that leaf morphology is an unreliable indicator of pure or hybrid individuals in these elms Fig. The majority of hybrids in naturalized populations were F 1 progeny and not backcrosses, suggesting recurring hybridization between U.

Additional later-generation backcross progeny may exist given the time period involved since initial introduction, but more markers would be needed to evaluate this possibility. We did not observe any backcrossing toward U. The observed introgression biased toward one of the parental species, U. Naturalized U. Although the number of alleles per locus was lower than in the East Asian accessions, at least when hybrids were excluded, sample sizes per population were also smaller.

The relatively high heterozygosity and low levels of inbreeding reflect the fact that most elm species are self-incompatible Santini et al. The excess heterozygosity in some of the naturalized populations could occur if inbred individuals do not survive to adulthood as has been found in other plant species Herlihy and Eckert Although we observed significant levels of admixture within our naturalized US populations, there were also significant levels of differentiation among some of these populations.

Hybrids contributed 66 U. The larger number of alleles specific to U. Populations with hybrids had more alleles per locus and greater levels of heterozygosity, but the increase in heterozygosity is not surprising as the majority of hybrids are F 1 s and therefore heterozygous at all species-specific loci.

The presence of hybrids increased the level of genetic differentiation among naturalized populations for some pairwise comparisons while it decreased it or did not change it for others.

However, the change in level of genetic differentiation in our pairwise comparisons was not necessarily related to the proportion of hybrids in these populations. Our data suggest that hybridization strongly affects the level of genetic diversity observed within U. These two US populations were not only genetically very similar to each other, but they also resembled the accessions from East Asia PEA.

The program structure placed all three populations within the same genetic cluster while the F ST -measures calculated for pairwise comparisons of these populations were low. We observed little genetic differentiation between the East Asian accessions and US populations. The genetic similarity observed here and the high level of genetic diversity within both the US populations and East Asian accessions suggest a pattern of multiple introductions of U.

This interpretation does not support Webb that seeds from as few as eight trees made up the source of most plantings in the USA. However, wind dispersal of both pollen and seeds in this species promotes high levels of mixing within and among populations such that seeds from a few trees can contain a large proportion of the genetic diversity in a population Zalapa et al.

Moreover, the widespread hybridization observed in naturalized populations of U. Ulmus pumila is likewise invasive in Europe and hybridizes with another elm species Ulmus minor in Spain where naturalized hybrid populations occur. Ulmus pumila was introduced in Spain as early as the 16th century, so hybridization and the evolution of invasiveness may have a much longer history there.

By further increasing genetic diversity and creating novel genotypes, hybridization appears to have facilitated the evolution of invasiveness in a number of introduced species Ellstrand and Schierenbeck ; Vila et al. In that respect, U. Nonetheless, U. Coincidentally, we observed hybridization between U. The increased heterosis and the creation of novel genotypes created via hybridization may have helped facilitate the adaptation of U.

The authors thank the University of Wisconsin-Madison Herbarium and the many collaborators that contributed tissue for DNA analysis; Ana Bravo for her assistance with the genetic analysis; and Kurtis Cooper for helpful discussion. Additional Supporting Information may be found in the online version of this article:. Figure S1. Figure S2. Leaf phenotypes of Ulmuspumila and Ulmus rubra top and their natural hybrids bottom.

Leaf phenotypes are a poor indicator of cross-type. Appendix S1. A total of alleles from 13microsatellite loci detected in putative Ulmus pumila individuals collected throughout the United States from cultivated material, herbarium specimen and eight naturalized populations. Please note: Wiley-Blackwell are not responsible for the content or functionality of any supporting materials supplied by the authors.

Any queries other than missing material should be directed to the corresponding author for the article. National Center for Biotechnology Information , U. Journal List Evol Appl v. Evol Appl. Author information Article notes Copyright and License information Disclaimer.

Received Sep 23; Accepted Oct This article has been cited by other articles in PMC. Abstract Ulmus pumila is considered an invasive tree in 41 of the United States. Keywords: genetic diversity, hybridization, invasive species, Siberian elm, Ulmus pumila , Ulmus rubra.

Introduction Biological invasions by exotic species together with loss of wildlife habitat, pollution and land-use change all pose serious threats to biodiversity and increase the risk of species extinctions Sakai et al. The percent polymorphic loci P pooled across all populations was 0. At the individual population level, P varied from 0. Because only one population of R.

Genetic diversity within populations H e ranged from 0. Observed heterozygosity H o ranged from 0. Single-locus genotypic frequencies generally conformed to Hardy—Weinberg expectations.

After controlling for multiple comparisons, 12 of the chi-squared tests showed significant deviations. Total genetic diversity at the polymorphic loci H T averaged 0. The proportion of genetic diversity among populations G ST ranged from 0. Six of the 10 G ST values were significant in R. Genetic identities among the hybrid populations ranged from 0.

Genetic identities among all populations hybrids, R. The hybrids and R. This latter value was nearly identical to the value provided by Sherman-Broyles et al. Nei's statistics of genetic diversity for all polymorphic loci in the six R. Although the Ft. Pickett than in the populations studied by Sherman-Broyles et al.

The higher levels of genetic diversity at Ft. Pickett may be a function of population size. In contrast to Ft.

Pickett, which represents the largest known concentration of R. However, the levels of genetic variation reported in our study are fairly typical when compared to other species with similar life-history characteristics.

Genetic diversity in R. The proportion of genetic diversity among populations of R. These G ST values are much lower than the values reported previously for R. A probable explanation for the low level of differentiation at Ft. The genetic data presented here support Hardin and Philips' contention that hybridization can and does occur between R. It is difficult, however, to determine the overall frequency of hybridization from these data.

Populations HYB3, HYB4, and RM6 consist almost exclusively of individuals that are heterozygous at Idh2 for the alleles that are characteristic of the two species, indicating that these populations may largely consist of F 1 hybrid individuals.

This result suggests that hybridization has played a major role in the production of these three populations.

There are two possible explanations for this finding. First, hybridization may have occurred infrequently at these sites, producing populations consisting of a majority of pure R. Conversely, hybridization may have occurred relatively frequently at these sites, producing populations of advanced-generation hybrids that have backcrossed extensively to R.

The hybrid populations are nearly identical to R. When compared to R. This difference disappears when Idh2 is removed from the analysis, indicating that the similarity of the hybrids with R. Unfortunately these findings do not help to distinguish between relatively rare hybridization in a population consisting mainly of pure R. The complete absence of R. It appears that hybridization between the two species, while not unusual, is generally local in nature.

Rhus michauxii , like other rare endemics, may be susceptible to extinction due to a combination of factors. Although Ft. Pickett represents the largest known concentration of R.

In addition, recent work has suggested that inbreeding can significantly increase extinction risk in local populations Saccheri et al. While species with restricted ranges typically have low levels of genetic diversity, our results reveal that there is little evidence of inbreeding at Ft.

Finally, our results indicate that R. Due to the apparently localized nature of hybridization at Ft. Pickett, however, hybridization does not appear to be an immediate threat to the existence of R. The effect of natural hybridization on rare and endangered species ultimately depends on factors such as the initial frequency of the rare taxon, the rate of hybrid production, and the relative fitness of the resulting hybrid individuals Wolf et al. Future studies aimed at quantifying these parameters will provide critical data for predicting the long-term effect of hybridization on the survival of R.

The authors thank L. Boyte, M. Bucher, V. Emrick, R. Sims, and T. Southall for assisting with the field work, and M. Burke and E. Stacy for assistance in the lab.

This work was supported by a grant from the U. Fish and Wildlife Service. Allozyme diversity in plant species. Sunderland, MA: Sinauer Associates; 43— Factors influencing levels of genetic diversity in woody plant species. New For 6 : 95 — Hybridization in eastern North American Rhus Anacardiaceae. ASB Bull 32 : 99 — Holm S, A simple sequentially rejective multiple tests procedure. Scand J Stat 6 : 65 — Conservation genetics of bull trout in the Columbia and Klamath drainages.

Conserv Biol 7 : — Hybridization and the extinction of rare plant species. Conserv Biol 10 : 10 — Little EL, The Audubon Society field guide to North American trees.

New York: Alfred A. Observations of the genetic structure and mating system of ponderosa pine in the Colorado Front Range. Theor Appl Genet 51 : 5 — Ledyard Stebbins focused on hybridization as an important generator of genetic diversity Stebbins, In contrast, zoologists such as Theodosius Dobzhansky and Ernst Mayr considered animal hybrids to be "rare" or "exceptional," and they instead discussed hybridization as a negative selective agent that favored the strengthening of discrimination to maintain species Dobzhansky, ; Mayr, Since then, scientists have been able to uncover many examples of hybridization across several branches of life.

This evidence initially came from fossil or extant species morphology, but more recently, molecular tools have enhanced hybridization research in a wealth of biological systems Mallet, Over the last 50 years, the study of hybridization has yielded valuable insights, not only refining scientists' systematic understanding of the taxa involved, but also helping researchers understand those forces that limit hybridization, as well as how gene flow and recombination can act to generate novel haplotypes to facilitate adaptation Arnold, When different genomes come into contact, gene exchange is not necessarily homogeneous, because alleles of some genes disperse across species boundaries more easily than others.

This observation has been reported across many different organisms, and it is associated with both single genes and large genomic regions, such as whole chromosomes or chromosomal regions.

One well-studied example involves reduced gene flow associated with inversion events on whole chromosomes in the North American Drosophila species D. These co-occurring species are less than 1 million years diverged and differ by three chromosomal inversions reversals in a segment of the DNA sequence.

Despite their limited nucleotide divergence and genomic rearrangements, these species hybridize at a low level in nature. Moreover, when examined genetically, allelic exchange also known as introgression between the species appears highest away from inverted regions and lowest within inverted regions Noor et al.

Although there is still much to learn, this system suggests a role for inversions in the persistence of hybridizing species.

In addition to fruit flies, house mice have also been studied extensively for variance in gene exchange across the genome. A large house mouse hybrid zone is found in central Europe, and the Nachman lab at the University of Arizona has analyzed the spread of traits and alleles across this hybrid zone to understand the evolutionary forces at work Payseur et al.

Researchers hypothesize that differential gene flow across species boundaries is a pattern resulting from selection, in which the absence of selection on some traits or alleles between species results in gradual transitions of these traits across a hybrid zone. In contrast, ongoing selection to maintain species at other traits prevents the dispersal of alleles across species boundaries and is expected to produce a pattern of steep transitions of traits across a hybrid zone.

In house mice, researchers found that alleles near the center of the X chromosome exhibit steep transitions across the hybrid zone, and these alleles are in a region previously shown to be associated with hybrid sterility. This work demonstrates the utility of hybrid zones to test for the effect of selection on interspecies gene flow in particular regions of the genome.

By continuing to study hybridizing species and hybrid zones, researchers can further understand which sets of genes have permissible gene flow and which are more restricted. Then, by studying genes in the latter category, scientists can learn which genes or gene sets may be important in maintaining species integrity in the face of gene flow.

Although research in some systems has brought insights into the nature of hybridization, detecting and quantifying interspecies gene flow is surprisingly difficult. Until recent advances in sequencing technologies, scientists relied on phenotypic characters to study hybridization, which sometimes yielded results incongruent with molecular data. Furthermore, identification of hybrids is also difficult using phenotype alone because of the paucity of early generation hybrids in the wild and because introgression is not always apparent in phenotypes.

These limitations are now becoming obsolete as scientists have the ability to assess hybridization using neutral molecular markers. As with phenotypic characters, closely related species may share some molecular variation that persists after their initial split, making it difficult to distinguish this shared variation from gene flow after speciation, as explained by coalescent theory Hey, Additionally, phylogenetic estimates of species relationships often contrast with one another because forces such as drift, selection, and recombination affect different parts of the genome differently.

Thus, it is not surprising that with the advent of new high-throughput sequencing technologies and more powerful processors in the last decade, the number of studies focusing on testing hybridization between species has increased by orders of magnitude. Apart from the technological difficulties of studying hybridization are the philosophical problems associated with interpreting interspecies gene exchange.

One's perception of species is important if one is to describe processes across species boundaries. Thus, some biologists work their definitions of hybridization around species concepts Arnold, , while some species concepts allow for gene flow Mallet, Nonetheless, it is by overcoming these difficulties that scientists have been able to advance the study of hybridization in the last half century.

As scientists continue to explore the interface of species through hybrid zones and analysis of introgression, they also continue to discover the facets of genetic diversity that contribute to generating and maintaining the biodiversity of our living planet.

Arnold, M. Anderson's paradigm: Louisiana irises and the study of evolutionary phenomena. Molecular Ecology 9 , — Darwin, C. Dobzhansky, T. Excoffier, L. Computer programs for population genetics data analysis: A survival guide. Nature Reviews Genetics 7 , — doi Felsenstein, J. Accuracy of coalescent likelihood estimates: Do we need more sites, more sequences, or more loci? Molecular Biology and Evolution 23 , — Fredrickson, R.

Dynamics of hybridization and introgression in red wolves and coyotes. Conservation Biology 20 , —