Evidence of hybrid breakdown among invasive hybrid cattails (Typha × glauca)

Abstract

Interspecific hybridization has varied consequences for offspring fitness, with implications for the maintenance of species integrity. Hybrid vigour, when it occurs, can peak in first-generation (F1) hybrids and then decline in advanced-generation (F2+) hybrids. This hybrid breakdown, together with the processes affecting patterns of hybridization and hybrid fitness, determine the evolutionary stability of hybrid zones. An extensive hybrid zone in North America involving the cattails Typha latifolia, T. angustifolia, and their invasive hybrid T. × glauca is characterized by hybrid vigour among F1s, but the fitness of advanced-generation hybrids has not been studied. We compared seed germination and plant growth of T. latifolia (parental L), F1 T. × glauca (F1), hybrid backcrosses to T. angustifolia (bcA) and T. latifolia (bcL), and advanced-generation (F2) hybrids. Consistent with expectations under hybrid breakdown, we found reduced plant growth for F2 hybrids in comparison with F1s (plant height and above-ground biomass) and parental Ls (above-ground biomass). Backcrossed hybrids had intermediate measures of plant growth and bcLs were characterized by reduced seed germination in comparison with parental Ls. Hybrid breakdown could make the formation of F1s in North America finite because (1) hybridization among cattails is asymmetric, with T. angustifolia but not T. latifolia subject to genetic swamping, and (2) T. angustifolia is less common and subject to competitive displacement by F1s. Hybrid breakdown is therefore expected to reduce hybrid frequencies over time, contributing to the long-term maintenance of T. latifolia - the only native cattail in the study region.

Fig. 1. Patterns of seed production and plant growth for parental and hybrid cattails following hand crossing.

A Seed set (measured as the number of seeds per unit mass of fruit) for the parental cross, square-root transformed. B Germination rate, measured as the proportion of seeds that germinated. C Plant height measured as the tallest leaf per plant. D Total above-ground dry mass, square-root transformed. Boxes indicate the median (horizontal black line in each box), the first and third quartiles (the lower and upper edge of each box), and the minimum and maximum values that were no further than 1.5 the inter-quartile range (whiskers extending from each box) for each cross or offspring type. Data points show values for each replicate. Upper-case letters and codes below each box indicate taxon. In figure panel A, upper case letters refer to the maternal × paternal parent for each cross type, with A = Typha angustifolia; L = Typha latifolia; F1 = F1 hybrids. For panels B–D upper case letters refer to progeny classes, with L = the progeny of L × L crosses, F1 = the progeny of A × L crosses, F2 = the progeny of F1 × F1 crosses; bcA = the progeny of F1 × A crosses; and bcL = the progeny of F1 × L crosses. Lowercase letters above each box indicate the results of pairwise tests – crosses or offspring types with different letters are significantly different from each other.