Invasive Plants and Enemy Release: Evolution of Trait Means and Trait Correlations in Ulex europaeus

Abstract

Several hypotheses that attempt to explain invasive processes are based on the fact that plants have been introduced without their natural enemies. Among them, the EICA (Evolution of Increased Competitive Ability) hypothesis is the most influential. It states that, due to enemy release, exotic plants evolve a shift in resource allocation from defence to reproduction or growth. In the native range of the invasive species Ulex europaeus, traits involved in reproduction and growth have been shown to be highly variable and genetically correlated. Thus, in order to explore the joint evolution of life history traits and susceptibility to seed predation in this species, we investigated changes in both trait means and trait correlations. To do so, we compared plants from native and invaded regions grown in a common garden. According to the expectations of the EICA hypothesis, we observed an increase in seedling height. However, there was little change in other trait means. By contrast, correlations exhibited a clear pattern: the correlations between life history traits and infestation rate by seed predators were always weaker in the invaded range than in the native range. In U. europaeus, the role of enemy release in shaping life history traits thus appeared to imply trait correlations rather than trait means. In the invaded regions studied, the correlations involving infestation rates and key life history traits such as flowering phenology, growth and pod density were reduced, enabling more independent evolution of these key traits and potentially facilitating local adaptation to a wide range of environments. These results led us to hypothesise that a relaxation of genetic correlations may be implied in the expansion of invasive species.

Oviposition Decision of the Weevil Exapion ulicis on Ulex europaeus Depends on External and Internal Pod Cues

“…While further experiments would be necessary to precisely identify all of the cues involved, our results show that E. ulicis females use both internal and external cues in their oviposition choice, and that response to at least some of these cues is adaptive. However, the preferences observed here on isolated pods were different from those observed on isolated flowers, and did not explain the whole-plant infestation rate in the common garden [27]. Together with previous studies, these results reveal that the foraging behaviour of E. ulicis weevils requires the capacity to respond to a wide variety of cues: whole-plant traits [21], flower traits, internal and external pod cues (this study), and cues indicating the presence of conspecifics [26] or the presence of the competitor Cydia succedana [19].…”

“…To focus on pod and flower cues and avoid the confounding effects of whole-plant traits such as pod and flower density or plant size [22], we worked on fresh isolated pods and flowers that were collected just before the experiments in the common garden described by Hornoy et al [27]. In this garden, located in Rennes (Brittany, France), gorse plants from seeds collected in Brittany and Scotland (native range), and Reunion and New Zealand (invaded range) have been grown in a randomized pattern since October 2006.…”

“…Thus, to link weevil offspring performance to pod traits, we made measurements on the plants grown in the common garden. These were made on 103-106 plants (8-10 plants per population, 21-28 plants per region, see Hornoy et al [27] for details). For each gorse individual in the garden, 30 ripe pods were randomly chosen and opened.…”

Two colonisation stages generate two different patterns of genetic diversity within native and invasive ranges of Ulex europaeus

“…Population names were encoded with the first letters representing the region it comes from: SPA for Spain, FRA for France, SCO for Scotland, CHI for Chile, NZE for New Zealand, REU for Reunion, and USA for California, USA. Populations FRA1, FRA2 and FRA3 correspond to populations BCC, BCV and BKE, respectively, in Hornoy et al (2011Hornoy et al ( , 2012; SCO1, SCO2, SCO3 correspond to SBA, SCR, SST; REU1, REU2, REU3 correspond to RLB, RMA, RPB; and NZE1, NZE2, NZE3 correspond to ZAU, ZCH, ZWE, respectively.…”

“…Also, very low cytoplasmic diversity has been found in the Ulex genus (Cubas et al, 2005;Kader Ainouche, personal communication). However, it has been shown that the phenotypic diversity is great, both in the native range and in the invaded regions (Hill et al, 1991;Tarayre et al, 2007;Hornoy et al, 2011), and that it has a genetic basis (Atlan et al, 2010). The study of genetic diversity in allopolyploids like U. europaeus has now become possible using methods recently developed for nuclear markers, such as allozymes or microsatellites (Obbard et al, 2006;Falush et al, 2007).…”