Hybridization, Hybrid Fitness and the Evolution of Adaptations

Plant Biosystems, Vol. 142, No. 1, March 2008, pp. 166 – 171

HYBRIDS AND IRIS

Hybridization, hybrid ﬁtness and the evolution of adaptations

M. L. ARNOLD, R. S. CORNMAN & N. H. MARTIN

Department of Genetics, University of Georgia, USA

Abstract This review focuses on two related outcomes of hybridization sensu lato (i.e. findings from both natural and experimental hybrids are discussed): (1) the production of hybrid genotypes with various fitnesses, and (2) the origin/transfer of adaptations in the Louisiana Iris species complex. Since effects on fitness, and the origin or transfer of adaptations, are of fundamental evolutionary importance, the examples discussed in this review reflect some of the most significant phenomena deriving from the transfer of genetic material via natural hybridization.

Key words: Adaptation, hybridization, genetic exchange, reticulate evolution

Another type of evidence that facilitates inferences Introduction concerning the relative fitness of hybrid genotypes With regard to the topic of this review, Arnold and the role of adaptive trait origin or transfer derives (2006) discussed examples encompassing viruses, from experimental analyses (e.g. Rhode & Cruzan, bacteria, plants and animals—all of which reflected 2005). In this case, an array of hybrid and parental the acquisition of novel adaptations through genetic genotypes are placed into the same environmental exchange events. Significantly, these acquisitions setting (e.g. see Bolnick & Near, 2005 for a review of apparently increased the fitness of certain hybrid hybrid embryo viability in crosses between centrarch- genotypes, allowing them to occupy novel environ- id fish taxa). Fitness estimates are obtained and the ments. Adaptive genetic exchange can be inferred in hybrid and parental genotypes ranked relative to one multiple ways. First, inferences can be constructed another. The fitness estimates may then be used to through the examination of genetic markers across infer which hybrid genotypes demonstrate the natural hybrid zones, either alone or in concert with acquisition of adaptive traits from one parent (i.e. data concerning the habitat associations of various adaptive trait transfer) or from a combination of parental and hybrid genotypes (see Moore, 1971; genes from more than one parental taxon (i.e. the Endler, 1977; Arnold, 1997, 2006 for discussions evolution of novel adaptive traits). Experimental and additional references). These types of analyses investigations of hybrid fitness and adaptive trait test for (1) the expected and observed frequencies of origin/transfer have demonstrated a large variability parental and hybrid genotypes across contact zones in the number and category of fitness components to test for a significantly increased frequency of examined. For example, Burke et al. (1998a) introgressed markers, and/or (2) significant associa- collected data for both asexual and sexual fitness tions between certain genotypes and microhabitats. components for Iris fulva Ker Gawl., Iris hexagona Either class of observation is consistent with a Walter, and their F1 hybrids. In a separate analysis, hypothesis of adaptive trait evolution. Thus, if Burke et al. (1998b) estimated the fitness of F2 specific genomic regions introgress at a significantly hybrids formed in crosses between I. fulva and Iris greater frequency than predicted, or are associated brevicaulis Raf. by examining the genotypes of the F2 preferentially with certain novel habitats, they are progeny. In the latter study, fitness estimates were identified as possible adaptive trait loci. determined from the survivorship of only hybrid

Correspondence: Michael L. Arnold, Department of Genetics, Life Sciences Building, University of Georgia, Athens, Georgia 30602, USA. Tel.: 706-542-1407. Fax: 706-542-3910. E-mail: [email protected] ISSN 1126-3504 print/ISSN 1724-5575 online ª 2008 Societa` Botanica Italiana DOI: 10.1080/11263500701873018 Hybridization, hybrid fitness and adaptation 167 genotypes, rather than from data for both hybrids introgressive hybridization awaited the availability of and parental individuals (as in Burke et al., 1998a). discrete, molecular markers (see Arnold, 2006 for a However, both studies detected higher fitness for discussion). The development of the genetic markers some hybrid genotypes in the controlled greenhouse necessary to test for (1) introgressive hybridization in environment. This concordant finding supported the nature, (2) the effect of such introgression on the hypothesis that some genotypes had gained adaptive fitness of various hybrid genotypes, and (3) the origin traits that allowed an increase in their fitness— and/or transfer of adaptations through introgression relative to other hybrid and parental genotypes—in began in the early 1990s. The first of these analyses this environmental setting. supported Anderson’s contention (Anderson, 1948, In the following sections we will discuss a series of 1949) that the evolution of the Louisiana Iris species studies that define the role of introgressive hybridiza- had been influenced by introgression. Specifically, tion in the plant group known as the Louisiana Irises. some populations of I. fulva, I. brevicaulis and In particular, we will illustrate how introgression has I. hexagona possessed genetic markers from the apparently led to hybrid genotypes with increased alternate species (Arnold et al., 1990a,b; Nason fitness and the transfer and/or origin of adaptations et al., 1992). to novel habitats. The initial molecular analyses confirmed the occurrence of introgressive hybridization as an evolutionary force in the Louisiana Iris species Louisiana Irises—A paradigm for studies of complex. Recent studies have further clarified the introgressive hybridization role of introgression in determining the fitness and Edgar Anderson published his classic treatise, ecological adaptations demonstrated by hybrid in- Introgressive Hybridization, in 1949. In the first dividuals. In particular, various investigators have chapter of his book, Anderson used the morpholo- come to the conclusion that genetic exchange has gical variation found in naturally occurring parental resulted in the transfer of loci that affect the fitness of and hybrid populations of the plants known as hybrid genotypes across habitats. For example, Louisiana Irises (as reported by Riley, 1938) to Cruzan and Arnold (1993) and Johnston et al. illustrate the process of introgressive hybridization (2001) defined microhabitat associations within (i.e. ‘‘introgression’’; Anderson & Hubricht, 1938). natural hybrid zones between I. fulva, I. brevicaulis Anderson had earlier (1948) utilized this same and I. hexagona. Results from these analyses reflected species complex to exemplify the effect of human- diagnostic, environmental associations for some of mediated, ecological disturbances on introgression. the parental and hybrid genotypic classes. Signifi- In both his paper and book, Anderson indicated the cantly, one hybrid class was found to occur in a novel primary significance of natural hybridization in the habitat relative to the parental individuals and other evolutionary history of Louisiana Iris species. Speci- hybrid genotypic classes as well (Cruzan & Arnold, fically, he concluded that natural hybridization had 1993). This observation was consistent with the resulted in genetic exchange in the form of intro- hypothesis that adaptations to novel microhabitats gression. Furthermore, Anderson held that the could evolve through the combining of parental, fundamentally important role of introgression lay in genomic regions in hybrid progeny. its ability to greatly increase the ‘‘variation in the participating species . . .’’ and thus ‘‘ far outweigh the Louisiana Irises and the origin/transfer of immediate effects of gene mutation’’ (Anderson, adaptations 1949: 61 – 62). In spite of the potential for genetic exchange to More recent analyses of the spatial distribution of affect evolutionary change, Anderson and Hubricht genotypes and paternity in natural hybrid zones (1938) discussed the difficulty in detecting introgres- along with linkage and QTL (quantitive trait locus) sion when using quantitative (e.g. morphological) mapping of fitness components have added further traits such as those utilized by Riley (1938) and support to the conclusions of Anderson (1949), Anderson (1949). Although such transfer might Cruzan and Arnold (1993) and Johnston et al. result in evolutionarily novel and important (2001). For example, Cornman et al. (2004) deter- genotypes (e.g. see Anderson & Hubricht, 1938; mined the spatial distribution of different hybrid Anderson, 1949; Anderson & Stebbins, 1954; clones in a naturally occurring hybrid zone between I. Lewontin & Birch, 1966; Arnold, 1997, 2006; fulva, I. brevicaulis and I. hexagona. In addition, they Chiba, 2005), the ‘‘wider spread of a few genes . . . determined the contribution to paternity by various might well be imperceptible’’ (Anderson, 1949: 102) genotypes within the hybrid zone and by plants when quantitative traits were the markers for gene outside the study area. Their findings—of significant transfer. In Louisiana Irises, as in taxa as diverse as spatial genetic structure and recruitment favoring protozoans, plants and animals, rigorous tests for certain hybrid genotypes—were consistent with the 168 .L rode al. et Arnold L. M.

Figure 1. Frequencies of introgressed alleles segregating in (A) I. brevicaulis Raf. and (B) I. fulva Ker Gawl. BC1 hybrids. Each linkage group (i.e. ‘‘1 – 22’’) is represented by a graph and genetic distances are given on the x-axis in Kosambi centimorgans. A scale of the frequencies of the introgressed alleles is given on the y-axis (expected value is 0.50). Frequencies greater than 0.50 indicate an overrepresentation of the introgressed alleles. Frequencies less than 0.50 reflect an underrepresentation of the introgressed alleles. Marker transmission ratio distortion as evaluated by w2 tests is indicated by an open circle (nonsignificant), 6 (a 5 0.05), and a solid circle (a 5 0.01). For instances where two or more contiguous markers demonstrated significant transmission ratio distortion, the Bayesian posterior probability of a transmission ratio distorting locus (i.e. TRDL) is given in italics and the posterior distribution of the TRDL location is shown as an accompanying histogram (Bouck et al., 2005). Hybridization, hybrid fitness and adaptation 169 hypothesis of hybrid genotypes having a higher fitness house were watered regularly, this environment due to acquired adaptations (Cornman et al., 2004). reflects a water-limited setting for some of the Indeed, these authors argued that selection for certain genotypes. In particular, although I. brevicaulis is hybrid genotypes and against other hybrid progeny often found in dryer, greenhouse-like, natural could lead to ‘‘the establishment of recombinant environments, wild populations of I. fulva most lineages that are more fit than the parental types in often occur in water-saturated soils (Viosca, 1935; some habitats’’ (Cornman et al., 2004). Cruzan & Arnold, 1993; Johnston et al., 2001). One A recent series of genetic mapping analyses have expectation derived from the observations of habitat also underlined the importance of introgressive associations in the natural populations of these hybridization in the origin and evolution of adapta- species is that alleles from I. brevicaulis (i.e. the tions to various environments. Figure 1 illustrates ‘‘dry-adapted’’ species) should increase survivorship results from a study by Bouck et al. (2005). Their in the relatively dry greenhouse environment. study defined genomic regions, in both I. brevicaulis Consistent with the above prediction, differences and I. fulva backcross (BC1) individuals, which were in mortality were found between the two backcross characterized by either significantly lower- or higher- (BC1) hybrid classes (originally studied by Bouck than-expected levels of introgression. In regard to the et al., 2005), with I. fulva backcrosses demonstrating current discussion, the observation of a significantly twice the frequency of mortality as I. brevicaulis increased frequency of introgression likely reflects backcross plants. Furthermore, mapping analyses positive selection for regions that contributed to detected four QTLs in the I. fulva hybrids that were increased survivorship in the greenhouse environ- significantly associated with survivorship (Martin ment. Similarly, Martin et al. (2005) defined QTLs et al., 2005). As expected, introgressed I. brevicaulis that were significantly associated with, and thus DNA increased survivorship at three of the four inferred to have affected, long-term survivorship in QTLs. Surprisingly, the fourth QTL indicated that the greenhouse environment (Figure 2). Although such introgression was associated with decreased the Louisiana Iris plants propagated in the green- survivorship. In this latter case, the presence of two

Figure 2. Linkage groups derived from BC1 hybrids toward I. fulva Ker Gawl. The figures indicate the proportion of individuals surviving that are heterozygous (having one introgressed I. brevicaulis Raf. allele) and homozygous (having two recurrent I. fulva alleles) at each locus along the 22 linkage groups. Open boxes indicate survivorship of heterozygotes while filled boxes indicate survivorship of homozygotes. Significant, single-marker tests are denoted with asterisks (*P 5 0.05, **P 5 0.01, ***P 5 0.001). The locations of QTLs plus 2-lod confidence intervals are identified at the top of the linkage groups with a vertical and horizontal bar, respectively (Martin et al., 2005). 170 M. L. Arnold et al. copies of the I. fulva genomic region increased may help to explain patterns of genetic variation survivorship (Martin et al., 2005). Although observed in naturally occurring hybrid zones’’ unexpected, this result indicates the adaptive poten- (Martin et al., 2006). tial that derives from combining genomic elements from different evolutionary lineages, in hybrid Acknowledgements individuals. In light of their results, Martin et al. (2005) The Louisiana Iris research has been supported by concluded the following: numerous grants. The most recent funding has come from the National Science Foundation (DEB- The present findings have important implications for the 0345123). evolutionary dynamics of naturally occurring hybrid zones. Regions of the genome that increase survivorship References when in a heterozygous (i.e., hybrid) state should have an increased likelihood of passing across species Anderson E. 1948. Hybridization of the habitat. Evolution 2:1 – 9. Anderson E. 1949. Introgressive hybridization. New York: John boundaries, whereas those that decrease survivorship Wiley and Sons, Inc. will be less likely to introgress. Anderson E, Hubricht L. 1938. Hybridization in Tradescantia. III. The evidence for introgressive hybridization. Am J Bot A subsequent investigation strengthened this con- 25:396 – 402. clusion. In this latter study, Martin et al. (2006) Anderson E, Stebbins GL, Jr. 1954. Hybridization as an evolutionary stimulus. Evolution 8:378 – 388. again utilized genomic surveys including QTL Arnold ML. 1997. Natural hybridization and evolution. Oxford: mapping. However, unlike either Bouck et al. Oxford University Press. (2005) or Martin et al. (2005), their analyses Arnold ML. 2006. Evolution through genetic exchange. Oxford: involved genotypes grown under natural conditions. Oxford University Press. Furthermore, in contrast to the relatively dry green- Arnold ML, Bennett BD, Zimmer EA. 1990a. Natural hybridization between Iris fulva and I. hexagona: pattern of ribosomal house setting of the first two experiments, the DNA variation. Evolution 44:1512 – 1521. genotypes in the Martin et al. (2006) analysis were Arnold ML, Hamrick JL, Bennett BD. 1990b. Allozyme variation exposed to a flood event. The following observations in Louisiana Irises: A test for introgression and hybrid were made from this latter study: speciation. Heredity 65:297 – 306. Bolnick DI, Near TJ. 2005. Tempo of hybrid inviability in centrarchid fishes (Teleostei: Centrarchidae). Evolution (1) I. fulva individuals survived at significantly 59:1754 – 1767. higher frequencies than I. brevicaulis plants; Bouck AC, Peeler R, Arnold ML, Wessler SR. 2005. Genetic (2) I. fulva backcrosses had a significantly higher mapping of species boundaries in Louisiana Irises using frequency of survivorship than the reciprocal IRRE retrotransposon display markers. Genetics 171:1289 – backcross toward I. brevicaulis; 1303. Burke JM, Carney SE, Arnold ML. 1998a. Hybrid fitness in the (3) survivorship of the I. brevicaulis BC1 hybrids Louisiana Irises: Evidence from experimental analyses. Evolu- was influenced by the presence of introgressed tion 52:37 – 43. I. fulva alleles located throughout the genome; Burke JM, Voss TJ, Arnold ML. 1998b. Genetic interactions and and natural selection in Louisiana Iris hybrids. Evolution 52:1304 – (4) survivorship in the I. fulva BC hybrids was 1310. 1 Chiba S. 2005. Appearance of morphological novelty in a hybrid significantly affected by two epistatically acting zone between two species of land snail. Evolution 59:1712 – QTL of opposite effects (Martin et al., 2006). 1720. Cornman RS, Burke JM, Wesselingh RA, Arnold ML. 2004. With regard to point (4), it is intriguing that the two Contrasting genetic structure of adults and progeny in a QTLs that affected survivorship in the natural setting Louisiana Iris hybrid population. Evolution 58:2669 – 2681. Cruzan MB, Arnold ML. 1993. Ecological and genetic associa- were on two of the linkage groups that contained tions in an Iris hybrid zone. Evolution 47:1432 – 1445. QTLs that also impacted survivorship in the dry Endler JA. 1977. Geographic variation, speciation, and clines. greenhouse environment. However, the effects were Princeton, NJ: Princeton University Press. in opposite directions—i.e. the introgressed QTLs in Johnston JA, Wesselingh RA, Bouck AC, Donovan LA, one setting lowered survivorship while in the other Arnold ML. 2001. Intimately linked or hardly speaking? The relationship between genotypic variation and environmental setting they increased survivorship (Martin et al., gradients in a Louisiana Iris hybrid population. Mol Ecol 2006). 10:673 – 681. Taken together, the above studies indicate broadly Lewontin RC, Birch LC. 1966. Hybridization as a source of based support for Anderson’s (1949) conclusion that variation for adaptation to new environments. 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