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.

Data collection

The data in the corresponding paper involved two phases: (1) hand crosses conducted in natural populations of cattails; (2) the germination and growth of seedlings arising from the hand crosses. Crosses were conducted with the aim of creating de novo F₁ hybrids (T. latifolia x T. angustifolia), F₂ hybrids (F₁ x F₁), the two types of back-cross hybrids both with F₁s as the maternal parent (backcross to T. angustifolia, bcA; backcross to T. latifolia, bcL). Seed set, germination and growth of these plants was compared to that of T. latifolia, also generated from hand crosses (T. latifolia x T. latifolia). Seed set was estimated using a subsample of the fruit. Germination was assessed in petrie dishes placed in a greenhouse. Seedlings were transplanted into pots and grown in the same greenhouse. After 90 days of growth plant height and above-ground biomass were measured for each seedling.

Data processing

Data processing and analysis were conducted using R and the scripts used are included alongside the data.

We used R to analyse the data, however, the files are in txt and csv format and can be opened using a variety of proprietary and open-source alternatives.