Inbreeding depression and heterosis vary in space and time in the serpentinophyte perennial Minuartia smejkalii

Heterosis has been used occasionally in attempts to save endangered populations of plants. However, there is a lack of knowledge on how inbreeding, heterosis and outbreeding depression jointly influence fitness of species with small, fragmented populations. Understanding the joint action of these processes is further complicated by variation among populations and environments in time and the stochastic effects of genetic drift. We assayed offspring of hand pollinated plants from three natural populations of Minuartia smejkalii, an endemic serpentinophyte perennial, in two soil treatments and three competitive environments for two consecutive years. We detected no early-acting inbreeding or outbreeding depression in a greenhouse. Late-acting inbreeding depression in the common garden was low in small populations and could be counteracted by heterosis. Both inbreeding depression and heterosis varied among traits, between years and with environmental stress. Notably inbreeding depression declined as environmental stress increased and phenotypic variation in the population decreased. Moreover, heterosis increased with environmental stress. Based on our results, we recommend a conservation approach in which between-population outbred plants are introduced into very small populations to maximise the benefits of heterosis in M. smejkalii. Nevertheless, extrapolation to other species, or even other populations of M. smejkalii of limited size, should be done with caution because of the stochastic effects of genetic drift that result in unique genetic consequences of outbreeding for each population.