Loci Affecting Long-Term Hybrid Survivorship in Louisiana Irises: Implications for Reproductive Isolation and Introgression

Abstract

Abstract. Iris fulva and I. brevicaulis are long-lived plant species known to hybridize where they coexist in nature. Year-to-year survival contributes significantly to overall fitness for both species and their hybrid derivatives, and differences in hybrid survivability may have important consequences to interspecific gene flow in nature. We examined the genetic architecture of long-term survivorship of reciprocal backcross I. fulva ϫ I. brevicaulis hybrids in a commongarden, greenhouse environment. Differences in mortality were found between the two backcross (BC 1 ) hybrid classes, with hybrids crossed toward I. fulva (BCIF) revealing twice the mortality of those hybrids backcrossed toward I. brevicaulis (BCIB). Using genomic scans on two separate genetic linkage maps derived from the reciprocal hybrid populations, we found that hybrid survivorship is influenced by several genetic regions. Multiple interval mapping (MIM) revealed four quantitative trait loci (QTLs) in BCIF hybrids that were significantly associated with survivorship. Introgressed I. brevicaulis DNA increased survivorship at three of the four QTLs. For the fourth QTL, introgressed I. brevicaulis DNA was associated with decreased survivorship. No QTLs were detected in BCIB hybrids; however, single-marker analysis revealed five unlinked loci that were significantly associated with survivorship. At all five markers, survivorship was positively associated with introgressed I. fulva DNA. The present findings have important implications for the evolutionary dynamics of naturally occurring hybrid zones. Regions of the genome that increase survivorship when in a heterozygous (i.e., hybrid) state should have an increased likelihood of passing across species boundaries, whereas those that decrease survivorship will be less likely to introgress.

Hybrid fitness, adaptation and evolutionary diversification: lessons learned from Louisiana Irises

“…With regard to Louisiana Irises, several studies that defined genotypic variation have paved the way for inferences regarding hybrid fitness and its possible relationship with adaptive evolution and lineage diversification (Bouck et al, , 2007Martin et al, 2005Martin et al, , 2006Martin et al, , 2008Taylor et al, 2009;Tang et al, 2010). Each of these studies involved the application of linkage and/or quantitative trait loci (QTL) mapping methodologies and thereby allowed inferences regarding the number and position of genes contributing to the fitness of hybrids.…”

“…I. fulva typically grows at lower elevations along the bayou edges, where rhizomes are often submerged in water, whereas I. brevicaulis generally occurs at slightly higher elevations in mixed hardwood forest (Figure 2a) (Viosca, 1935;Cruzan and Arnold, 1993;Johnston et al, 2001b). Martin et al (2005Martin et al ( , 2007 have tested for QTL underlying survivorship in different habitats. These analyses involved I. fulvaÂI.…”

“…The first experiment examined the genetic architecture associated with the survivorship of hybrid genotypes in a greenhouse setting (Figure 2b) (Martin et al, 2005). Although well watered in this common garden experiment, BC 1 iris genotypes were not placed in standing water as is typical for I. fulva populations.…”

“…For example, in the relatively dry greenhouse environment, (1) alleles from 'wet-adapted' I. fulva increased the fitness of introgressed I. brevicaulis genotypes (Martin et al, 2005); (2) I. fulva alleles marked a QTL associated with higher survivorship of I. fulva BC 1 genotypes (Martin et al, 2005); and (3) although I. fulva BC 1 genotypes died at twice the frequency of BC 1 hybrids toward I. brevicaulis, I. fulva alleles introgressed into I. brevicaulis at a significantly greater frequency than expected under a neutral model . In the flooded environment (Martin et al, 2006), one of the two QTLs associated with increased survivorship of introgressed I. fulva (that is, BC 1 genotypes toward I. fulva) was associated with an allele from 'dryadapted' I. brevicaulis.…”

“…The data available for these reviews (particularly those deriving from genomic information) have multiplied exponentially over the past decade due largely to the advances in methodologies for collecting and analyzing DNA sequences (Green et al, 2010) and for deciphering the function of the underlying genes (Rosas et al, 2010). However, the species complexes that have provided the most rigorous and detailed descriptions of hybrid fitness have done so because of the availability of both genetic and ecological information (Grant andGrant, 1992, 2010;Rieseberg et al, 2003;Martin et al, 2005Martin et al, , 2006 and see for a review). These various catalogs of hybrid fitness estimates have demonstrated that 'all hybrids are not created equal' in terms of their fitness.…”