The genetic architecture of reproductive isolation in Louisiana irises: flowering phenology

Abstract

Despite the potential importance of divergent reproductive phenologies as a barrier to gene flow, we know less about the genetics of this factor than we do about any other isolating barrier. Here, we report on the genetic architecture of divergent flowering phenologies that result in substantial reproductive isolation between the naturally hybridizing plant species Iris fulva and I. brevicaulis. I. fulva initiates and terminates flowering significantly earlier than I. brevicaulis. We examined line crosses of reciprocal F1 and backcross (BC1) hybrids and determined that flowering time was polygenic in nature. We further defined quantitative trait loci (QTL) that affect the initiation of flowering in each of these species. QTL analyses were performed separately for two different growing seasons in the greenhouse, as well as in two field plots where experimental plants were placed into nature. For BCIF hybrids (BC1 toward I. fulva), 14 of 17 detected QTL caused flowering to occur later in the season when I. brevicaulis alleles were present, while the remaining 3 caused flowering to occur earlier. In BCIB hybrids (BC1 toward I. brevicaulis), 11 of 15 detected QTL caused flowering to occur earlier in the season when introgressed I. fulva alleles were present, while the remaining 4 caused flowering to occur later. These ratios are consistent with expectations of selection (as opposed to drift) promoting flowering divergence in the evolutionary history of these species. Furthermore, epistatic interactions among the QTL also reflected the same trends, with the majority of epistatic effects causing later flowering than expected in BCIF hybrids and earlier flowering in BCIB hybrids. Overlapping QTL that influenced flowering time across all four habitat/treatment types were not detected, indicating that increasing the sample size of genotyped plants would likely increase the number of significant QTL found in this study.

Figure 1.—

Flowering phenology for experimental I. fulva (red), BCIF (pink), F₁ (yellow), BCIB (purple), and I. brevicaulis (blue) assessed on a daily basis at two separate sites. The census number (y-axis) is expressed here as a percentage of the maximum number of flowers open throughout the census period. (A) Dry site. A total of 146 I. fulva, 2005 BCIF, 470 F₁, 2115 BCIB, and 51 I. brevicaulis flowers were censused throughout the entire flowering period. (B) Wet site. A total of 233 I. fulva, 2351 BCIF, 489 F₁, 2469 BCIB, and 151 I. brevicaulis flowers were censused throughout the entire flowering period.

Figure 2.—

Observed vs. expected mean flower initiation dates (±2 SE) for I. fulva, I. brevicaulis, F₁, BCIF, and BCIB hybrids. The diagonal line denotes the expected mean flowering date under an additive model of gene action.

Figure 3.—

Linkage map of dominant I. brevicaulis IRRE retrotransposon display markers segregating in the F₁ hybrid used to produce BCIF hybrids. Significant QTL for flowering phenology are denoted (with 2-LOD confidence intervals) to the right of the linkage groups. Red bars represent regions where introgressed I. brevicaulis alleles caused flowering to initiate earlier, while blue bars represent regions where introgressed I. brevicaulis alleles caused flowering to initiate later. QTL analyses were performed during the two greenhouse years 2002 and 2003 as well as in two field plots (dry and wet) in 2006.

Figure 4.—

Linkage map of dominant I. fulva IRRE retrotransposon display markers segregating in the F₁ hybrid used to produce BCIB hybrids. Significant QTL for flowering phenology are denoted (with 2-LOD confidence intervals) to the right of the linkage groups. Red bars represent regions where introgressed I. fulva alleles caused flowering to initiate earlier, while blue bars represent regions where introgressed I. fulva alleles caused flowering to initiate later. QTL analyses were performed during the two greenhouse years 2002 and 2003 as well as in two field plots (dry and wet) in 2006.