Minimal-Risk Seed Heteromorphism: Proportions of Seed Morphs for Optimal Risk-Averse Heteromorphic Strategies

Abstract

Seed heteromorphism is the reproductive strategy characterized by the simultaneous production of multiple seed types. While comparing heteromorphic to monomorphic strategies is mathematically simple, there is no explicit test for assessing which ratio of seed morphs minimizes fitness variance, and hence offers a basis for comparing different heteromorphic strategies. Such a test may be particularly valuable when more than two distinct morphs are present, since many strategies may have equivalent geometric fitnesses. As noted by Gillespie (1974), in these cases avoiding rare but evolutionarily important instances of severe reductions in fitness involves the minimization of variation in fitness—i.e., risk. Here I compute the optimal proportions of two or more seed morphs for heteromorphic strategies that either: (1) minimize total fitness variance; or (2) maximize the fitness-risk ratio—i.e., the “extra” fitness accrued per unit of “extra” fitness variance. This work thereby provides a testable null hypothesis to estimate the optimal frequencies of seed morphs when multiple heteromorphic strategies have evolved in environments with severe fitness risks. Moreover, it also permits the calculation of expected seed morph frequencies when more than two seed morphs are produced.

Phenotypic plasticity in diaspore production of a amphi-basicarpic cold desert annual that produces polymorphic diaspores

“…Consequently, with an increase in favorable conditions for growth and survival of her offspring the mother plant devotes proportionally more resources to the high-risk morph. As such, then, production of dispersal unit morph a is a "timid" strategy for ensuring reproduction, while production of dispersal unit morph f is a "bold" strategy 47,48 . Moreover, production of dispersal unit morph c is a transitional type between "timid" and "bold" strategies.…”

Achene heteromorphism in Bidens pilosa (Asteraceae): differences in germination and possible adaptive significance

“…Many plant species inhabiting unpredictable environments have evolved a number of morphological and physiological adaptations to increase survival and fitness (Harper et al 1970; Hughes 2018). Bidens pilosa is a serious weed species that often disturbed by human activity.…”

Maternal effects on seed heteromorphism: a dual dynamic bet hedging strategy

“…Seed heteromorphism often is considered to be a typical example of adaptive bet hedging in a highly variable environment (Hughes, 2018). According to the model for the ecology and evolution of seed heteromorphism developed by Venable (1985), there are two kinds of seed heteromorphism: high-risk–low-risk (one morph with a high mean fitness but high risk of reproductive failure and another morph with a low mean fitness and a low risk of failure) and high-risk–high-risk (high mean fitness and high risk of reproductive failure for each seed morph).…”

“…According to the model for the ecology and evolution of seed heteromorphism developed by Venable (1985), there are two kinds of seed heteromorphism: high-risk–low-risk (one morph with a high mean fitness but high risk of reproductive failure and another morph with a low mean fitness and a low risk of failure) and high-risk–high-risk (high mean fitness and high risk of reproductive failure for each seed morph). Production of two or more seed types with different ecological strategies by a single individual is adaptive via bet hedging in an unpredictable environment because among-generation variance of reproductive success is reduced, and thus the geometric mean fitness is increased (Crean and Marshall, 2009; Sadeh et al ., 2009; Simons, 2011; Hughes, 2018). Variation in seed morph ratio reflects the prediction of plants to good/bad years and the factors that influence this variation include the phenotype of maternal plant, nutrient availability, competition and others (Venable, 1985; Wang et al , 2012; Doudová, 2017).…”