Evolution of enhanced reproduction in the hybrid-derived invasive, California wild radish (Raphanus sativus)

Abstract

Evolution is receiving increased attention as a potentially important factor in invasions. For example, hybridization may have stimulated the evolution of invasiveness in several well-known plant pests. However, the mechanism for success of such hybrid-derived lineages remains unknown in the majority of the cases studied. Here we ask whether increased reproductive success (in terms of maternal fitness) has evolved in an invasive lineage with confirmed hybrid ancestry. We compare the relative fitness of the non-native, hybrid-derived California wild radish (Raphanus sativus) to that of its two progenitor species in field experiments at different sites and in different years. We found that California wild radish has high survivorship and produces more fruits per plant and more seeds per plant than either of its progenitors in several environments. Furthermore, populations of California wild radish display a strong genotype-by-environment interaction, indicating that maintenance of genetic and phenotypic diversity between populations may be responsible for the weed's ability to invade a wide breadth of California habitats. Our results suggest that hybridization may contribute the evolution of enhanced invasiveness and, also, that by limiting the introduction and subsequent hybridization of congeners, we may be able to prevent the evolution of new invasive lineages.

Assessing the effects of hybridization and precipitation on invasive weed demography using strength of selection on vital rates

“…For instance, fecundity contributed positively to higher hybrid population invasiveness relative to wild populations only when precipitation was altered and not under the control shelter treatment. This work, where only soil moisture varied, supports previous work that has documented the context dependency of hybrid genotypes across North America, where many environmental variables differed among locations [3, 72]. However, we may have seen differences in λ among treatments if there been a larger seed-bank sample size [55] or if the watering treatments were extended to winter months.…”

The red queen in the corn: agricultural weeds as models of rapid adaptive evolution

“…California wild radish is notable in that it behaves as both an agricultural weed and occupant of natural areas. It displays higher fitness than both of its progenitors as determined by average fruit weight, rate of survival and reproductive output when grown in common gardens (Ridley and Ellstrand, 2009). These traits were likely acquired from the hybridization event itself; however, as the weeds are obligate outcrossers, heterosis, rather than particular wild or crop alleles, could also provide important contributions to weed fitness (Ridley and Ellstrand, 2009).…”

“…It displays higher fitness than both of its progenitors as determined by average fruit weight, rate of survival and reproductive output when grown in common gardens (Ridley and Ellstrand, 2009). These traits were likely acquired from the hybridization event itself; however, as the weeds are obligate outcrossers, heterosis, rather than particular wild or crop alleles, could also provide important contributions to weed fitness (Ridley and Ellstrand, 2009). This has been further supported by the construction of artificial radish hybrids which show elevated fitness in common garden experiments (Campbell et al, 2006).…”

Modes of reproduction in three invasive milkweeds are consistent with Baker’s Rule

“…Hybridization can potentially have beneficial consequences for invasiveness if, for example, it increases the vigour of progeny, creates genotypes that are more reproductively successful, increases genetic variation that enables faster adaptation and/or increases the pollen supply in sparse populations (e.g. see Ellstrand and Schierenbeck 2006;Scofield and Schultz 2006;Ayres et al 2008;Ridley and Ellstrand 2009;Meyerson et al 2010). Alternatively, inter-species visits by pollinators may be wasteful of pollen and/or ovules if they do not lead to viable seeds or if they result in unfit offspring due to genetic incompatibilities.…”