Effect of Whole-Genome Duplication on the Evolutionary Rescue of Sterile Hybrid Monkeyflowers

Abstract

Hybridization is a creative evolutionary force, increasing genomic diversity and facilitating adaptation and even speciation. Hybrids often face significant challenges to establishment, including reduced fertility that arises from genomic incompatibilities between their parents. Whole-genome duplication in hybrids (allopolyploidy) can restore fertility, cause immediate phenotypic changes, and generate reproductive isolation. Yet the survival of polyploid lineages is uncertain, and few studies have compared the performance of recently formed allopolyploids and their parents under field conditions. Here, we use natural and synthetically produced hybrid and polyploid monkeyflowers (Mimulus spp.) to study how polyploidy contributes to the fertility, reproductive isolation, phenotype, and performance of hybrids in the field. We find that polyploidization restores fertility and that allopolyploids are reproductively isolated from their parents. The phenotype of allopolyploids displays the classic gigas effect of whole-genome duplication, in which plants have larger organs and are slower to flower. Field experiments indicate that survival of synthetic hybrids before and after polyploidization is intermediate between that of the parents, whereas natural hybrids have higher survival than all other taxa. We conclude that hybridization and polyploidy can act as sources of genomic novelty, but adaptive evolution is key in mediating the establishment of young allopolyploid lineages.

Keywords: Erythranthe; Mimulus; allopolyploid; polyploidy; speciation; whole-genome duplication.

Figure 1

Diagram of the Experimental Design Used to Generate Synthetic Hybrid (Triploid, ABC) and Allohexaploid (AABBCC) Monkeyflowers (Mimulus spp.). Five populations were used for each of the parental species, diploid M. guttatus (AA) and the ancient allotetraploid M. luteus sensu lato (BBCC). For M. guttatus, we included three introduced populations in the British Isles (COL, DBL, and HOU) and two native populations (DUN and LMC). For M. luteus s.l. we included two introduced populations from the British Isles (COL and EVI), two native populations of two varieties (M. luteus var. luteus [EY] and M. luteus var. variegatus [RC]), and an experimental hybrid between the two varieties (EY × RC). F₁ seeds were treated with colchicine and screened using flow cytometry to identify experimental polyploids, which were then brought to flower and self-fertilized to generate S₁ seeds. F₁ and S₁ seeds were used in all subsequent experiments. Population details are given in Supplemental Table 2.

Figure 2

Diagram Illustrating the Viability of Seeds Obtained from Crosses between Monkeyflowers (Mimulus spp.) with Different Ploidy and Genomic Composition (Subgenomes Shown with Different Capital Letters). Arrowheads indicate the recipient of pollen in each cross type. Seed viability was assessed as the proportion of germinated seeds and is qualitatively indicated with different degrees of arrow shading. The lightest shading corresponds to germination proportions less than or equal to 2%. Quantitative values for seed germination are given in Table 1.

Figure 3

Pollen Viability in Individual Products of Intra- or Inter-taxon Crosses. The nine synthetic allopolyploid lines (“M. peregrinus”) are shown in green. Natural allopolyploids (M. peregrinus) are shown in blue (populations LED and STR). The parental species are shown in purple (M. guttatus) and yellow (M. luteus). The dotted line represents the average fertility reported for M. × robertsii (Vallejo-Marin, 2012). Each data point represents the mean of individuals derived from the same cross (full-sib family). The labels on the x-axis show the codes of populations used in each cross and are detailed in Supplemental Table 2. Vertical bars indicate the 95% confidence intervals (CIs) estimated using bootstrapping.

Figure 4

Comparison of 16 Phenotypic Traits in Synthetic Triploid Hybrid (Robertsii.3x.syn) Monkeyflowers (Mimulus spp.) and Their Synthetic Allohexaploid Derivatives (Peregrinus.6x.syn) Measured in a Common Garden in the Greenhouse. The traits measured were: days to flowering (FLTI), plant height (PLAH), flowering node of the first flower (FLNO), corolla width (CORW), corolla height (CORH), corolla tube length (TUBL), throat opening (THRO), throat width (THRW), calyx length (CALL), pedicle length (PEDL), the width (LEAW) and length (LEAL) of the largest leaf, bract width (BRAW) and length (BRAL), anther-stigma distance (ASD), and stem thickness (STTH). All units are in millimeters except FLTI (days) and FLNO (flower number). Statistical significance of differences between the triploid and the allohexaploids was assessed by MANOVA. ∗P < 0.05; ∗∗P < 0.01; ∗∗∗P < 0.001.

Figure 5

Proportion of Mimulus spp. Individuals of Mimulus spp.Surviving in a Field Plot in Central Scotland to the End of the First Growing Season (Autumn 2016) and Over Winter to the Following Spring (Spring 2017). Triploid (3x) individuals of M. × robertsii (MR) were created synthetically by crossing M. guttatus (MG) (2x) and M. luteus (ML) (4x). Allohexaploid (6x) individuals of M. peregrinus (MP) were synthesized by treating the seeds from synthetic M. × robertsii with colchicine. The experiment included 843 individuals and 1796 flowers; detailed sample sizes are provided in Supplemental Table 3. The symbols indicate mean survival, and the vertical bars show 95% CIs obtained by bootstrapping.

Figure 6

Survival of Mimulus spp. in a Field Plot in the Southern Uplands of Scotland Near Leadhills. Individuals were planted as cuttings in the autumn of 2015, and survivorship was assessed at the beginning of the following summer (June 2016) and at the end of the summer (August 2016). The experiment contained 828 individuals (ramets) from 298 genotypes (genets) of eight taxa and encompassed parents (M. guttatus:2x and M. luteus:4x) and their autopolyploid derivatives (M. guttatus:4x and M. luteus:8x), as well as inter-specific hybrids before (M. × robertsii:3x) and after WGD (M. peregrinus:6x) (for sample size information per taxon, see Supplemental Table 4). Both triploid and allohexaploid hybrids were represented by both natural and synthetically created individuals. The symbols indicate mean survival, and the vertical bars show 95% CIs obtained by bootstrapping.