Limiting cheaters in mutualism: evidence from hybridization between mutualist and cheater yucca moths

Mutualisms are balanced antagonistic interactions where both species gain a net benefit. Because mutualisms generate resources, they can be exploited by individuals that reap the benefits of the interaction without paying any cost. The presence of such ‘cheaters’ may have important consequences, yet we are only beginning to understand how cheaters evolve from mutualists and how their evolution may be curtailed within mutualistic lineages. The yucca–yucca moth pollination mutualism is an excellent model in this context as there have been two origins of cheating from within the yucca moth lineage. We used nuclear and mitochondrial DNA markers to examine genetic structure in a moth population where a cheater species is parapatric with a resident pollinator. The results revealed extensive hybridization between pollinators and cheaters. Hybrids were genetically intermediate to parental populations, even though all individuals in this population had a pollinator phenotype. The results suggest that mutualisms can be stable in the face of introgression of cheater genes and that the ability of cheaters to invade a given mutualism may be more limited than previously appreciated

Experimental analysis of constitutive and induced defence in a plant–seed–predator system

1. The functional significance of candidate defence mechanisms, including fruit abortion, fruit morphology and seed provisioning of Silene latifolia (Miller) Kraus (Caryophyllaceae), was tested using experimental exposure to its seed predator Hadena bicruris Hufnagel (Noctuidae). This moth lays eggs in S. latifolia female flowers, and each larva consumes three to four fruits to achieve development. Specifically, we sought to explain why fruit walls are thicker in European plants compared with plants in the introduced range where the seed predator is absent, which suggests a defensive role for fruit wall. We also analysed the carbon and nitrogen content of the seeds of exposed and unexposed plants to investigate whether plants respond to moth attack by modifying seed provisioning. 2. Experimental exposure to moth eggs significantly increased fruit abortion. Fruits built after attack on exposed plants did not have significantly thicker fruit walls compared with fruits on control plants. The proportion of N and C within seeds was significantly, but only slightly, modified in exposed plants. 3. Fruits with thicker fruit walls were not significantly less profitable, nor did they require longer handling time when exposed to larvae. 4. Populations varied significantly in fruit wall thickness. When flowers were infested experimentally, we found significant among-family variation in larval growth, suggesting genetic (or maternal environmental) variation. 5. We conclude that fruit abortion is an important defence mechanism. Constitutive investment in fruit wall thickness does not provide a strong protection against secondary attack. Plants do not appear to respond to attack by modifying fruit wall thickness or seed provisioning in later fruits. Future studies may compare fruit abortion and palatability of European and American plants with their greater variation, to address whether relaxed selection away from the seed predator modifies defences to this natural enemy