Germ Banking: Bet‐hedging and Variable Release From Egg and Seed Dormancy

Abstract

Many species produce eggs or seeds that refrain from hatching despite developmental preparedness and favorable environmental conditions. Instead, these propagules hatch in intervals over long periods. Such variable hatch or germination tactics may represent bet-hedging against future catastrophes. Empiricists have independently recognized these approaches in diverse species. Terms such as seed banking, delayed egg hatching, and embryonic diapause have been used to describe these tactics, but connections between fields of study have been rare. Here we suggest a general term, germ banking, to incorporate all previous terms, unifying many seemingly disparate biological strategies under a single definition. We define the phenomenon of germ banking and use several biological examples to illustrate it. We then discuss the different causes of variation in emergence timing, delineate which constitute germ banking, and distinguish between germ banking and optimal timing of diapause. The wide-ranging consequences of germ banking are discussed, including modification of the age structure of a population, the alteration of microevolutionary dynamics, the migration of alleles from the past, the maintenance of genetic and species diversity, and the promotion of species coexistence. We end by posing questions to direct future research.

Environmental and Genetic Sources of Diversification in the Timing of Seed Germination: Implications for the Evolution of Bet Hedging

“…Observed seed germination variance is often explained as a diversification bet-hedging strategy whereby individuals minimize the likelihood of complete reproductive failure by paying a cost in the form of a reduction in expected fitness. Explaining seed trait variance has been, and continues to be, a major concern to those interested in life-history evolution because of the close association between seed traits and fitness (e.g., Cohen 1966;Janzen 1969;Smith and Fretwell 1974;Harper 1977;Marks and Prince 1981;Venable 1985;Kalisz 1986;Michaels et al 1988;Venable and Brown 1988;Westoby et al 1992;Philippi 1993;Rees 1997;Simons and Johnston 2000a;Galloway 2002;Donohue et al 2005;Evans and Dennehy 2005).…”

Evolution of alternative insect life histories in stochastic seasonal environments

“…2008). Entering dormancy (called diapause in insects) is a way of delaying maturation, resulting in “germ banking” (Evans and Dennehy 2005). The dormant part of a population acts as a buffer against environmental uncertainty and facilitates population (or genotype) survival if the nondormant individuals fail to reproduce (Phillippi 1993; Tuljapurkar and Istock 1993; Menu et al.…”

Inference of seed bank parameters in two wild tomato species using ecological and genetic data

“…Depending on extinction/recolonization rates and the type of group founding events (migrant vs. propagule pool), genetic differentiation among demes may generate a higher genetic diversity (N e ) for the metapopulation as a whole than predicted by the sum of all individuals across demes (10). The reproductive mode (seed or egg dormancy), which is often described as a bet-hedging strategy in plants (11), invertebrates (Daphnia and mosquitoes), and microorganisms (12) to buffer against environmental variability (6,13), also generates an increase of the N e compared with the N cs in two ways. First, low germination rates in the banks promote the storage of genetic diversity and the increase of N e compared with above-ground plant populations (6)(7)(8), although slowing down the rate of evolution (5) and coevolution (14).…”

“…Evidence for seed dormancy adaptation (11,13) and for the influence of seed banks on plant evolution over evolutionary time scales (16) is scarce because of the complex ecological experiments in the field needed to measure it (11,16) and the complex genetics of these traits (17,18). Moreover, evidence for seed banks to increase the observed molecular diversity compared with expectations from the above-ground census size (15,16) can be confounded by the spatial structuring of populations, where a significant part of the diversity may be attributable to population differentiation.…”