Abstract
It is often assumed that the negative effects of inbreeding on fitness (inbreeding depression, ID) are particularly strong under stressful conditions. However, ID may be relatively mild under types of stress that plant populations have experienced for a long time, because environment‐specific deleterious alleles may already have been purged. We examined the performance of open‐ and self‐pollinated progeny of the short‐lived calcareous grassland plant Anthyllis vulneraria under three intensities of each of five types of stress. Drought, nutrient deficiency, and defoliation were chosen as stresses typical for the habitat of origin, while shade and waterlogging were expected to be novel, unfamiliar stresses for A. vulneraria. The stresses reduced plant biomass by up to 91%, and the responses of the plants were mostly in line with the functional equilibrium hypothesis. There was significant ID in biomass (δ = 0.17), leaf chlorophyll content, and the number of root nodules of the legume, but the magnitude of ID was independent of the stress treatments. In particular, there was no significant interaction between inbreeding and the intensity of any stress type, and ID was not higher under novel than under familiar stresses. In addition, phenotypic plasticity in biomass allocation, leaf functional traits and in root nodulation of the legume to the various stress treatments was not influenced by inbreeding. Our findings do not support the common hypothesis of stronger ID under stressful environments, not even if the stresses are novel to the plants.
Highlights
The mating of closely related individuals can severely affect off‐ spring fitness, a phenomenon called inbreeding depression (ID; Charlesworth & Charlesworth, 1987; Darwin, 1876)
Estimating the strength of ID in natural populations is of interest for both conservation and evolutionary biology, because ID plays an important role in the extinction of populations (Gilpin & Soulé, 1986; Hedrick & Kalinowski, 2000) and in the evolution of mating systems (Charlesworth & Charlesworth, 2010)
We ad‐ dressed the following questions: (a) Does ID in A. vulneraria increase with the strength of novel stresses, but not with that of stresses typical for the habitat of A. vulneraria? (b) Does ID increase under conditions that increase the phenotypic variation or initial size dif‐ ferences among plants? (c) Does inbreeding affect functional traits involved in the stress response of A. vulneraria?
Summary
The mating of closely related individuals can severely affect off‐ spring fitness, a phenomenon called inbreeding depression (ID; Charlesworth & Charlesworth, 1987; Darwin, 1876). It has been proposed that ID should be relatively low under types of stress that a population has been exposed to for a long time (Cheptou & Donohue, 2011; Reed et al, 2012) This is expected because selection should eliminate alleles that have detrimental ef‐ fects on fitness in such “familiar” stressful environments (Agrawal & Whitlock, 2010; Bijlsma, Bundgaard, & Putten, 1999; Pemberton et al, 2017). Other studies did not find consistent effects of inbreeding on the response of plant functional traits to different environments (Murren & Dudash, 2012; O'Halloran & Carr, 2010; Schlichting & Levin, 1986). We ad‐ dressed the following questions: (a) Does ID in A. vulneraria increase with the strength of novel stresses, but not with that of stresses typical for the habitat of A. vulneraria? (b) Does ID increase under conditions that increase the phenotypic variation or initial size dif‐ ferences among plants? (c) Does inbreeding affect functional traits involved in the stress response of A. vulneraria?
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