Nine out of ten breeding systems agree: Self-incompatible strategies are disadvantageous in fragmented habitats

Background/Question/Methods

Approximately half of all plant species are self-compatible (SC), and can self-fertilize, while half are self-incompatible (SI) and require pollen transfer in order to set seed.   Self-incompatibility has evolved several times, and numerous variations exist.  Each of these breeding systems responds differently to the problem of maximizing mate availability while minimizing inbreeding risk.  Prior work has demonstrated that SC species are better able to persist in fragmented habitats, suggesting that breeding system may be a useful criterion for prioritizing conservation effort.  It is unknown, however, whether differences between self-incompatibility systems affect the persistence of fragmented SI populations.

I have developed an individual-based, spatially explicit computational model to simulate the effects of habitat loss on populations with a range of breeding systems.  Life-history traits are parameterized for Echinacea angustifolia (Asteraceae), a well-studied prairie forb.  Simulated populations were assigned one of ten breeding systems: gametophytic SI, sporophytic SI with one of seven dominance patterns at the self-incompatibility (S) locus, and two SC systems.  Each population replicate was established with > 1000 individuals, and subsequently reduced to a fragment containing 500, 100, 50, 10 or 5 individuals.  Populations were followed for 500 years post-fragmentation.

Results/Conclusions

Outputs of the model show that mate limitation adversely affects population persistence in small habitats.  Inbreeding depression was important in large habitats, but only when inbreeding was the result of selfing rather than mating between close relatives.  Extinction for SI replicates occurred at larger fragment sizes than for SC replicates.  Although SI replicates experience differing degrees of mate limitation, population outcomes did not vary among 7 of the 8 SI systems.  The most mate-limited SI system (sporophytic SI with co-dominance at the S-locus) consistently produced worse outcomes than other SI systems.  Because S-locus dominance is pervasive in natural sporophytic systems, however, this worst-case scenario is unrealistic.  Thus, managers should feel confident making conservation prioritizations according to whether a population has SI or SC without concern for the details of SI.