War of hormones over resource allocation to seeds: Strategies and counter-strategies of offspring and maternal parent

Dividing a Maternal Pie among Half-Sibs: Genetic Conflicts and the Control of Resource Allocation to Seeds in Maize

Resource allocation to offspring is the battleground for various intrafamilial conflicts. Understanding these conflicts requires knowledge of how the different actors (mother, siblings with different paternal genotypes) influence resource allocation. In angiosperms, allocation of resources to seeds happens postfertilization, and the paternally inherited genome in offspring can therefore influence resource allocation. However, the precise mode of resource allocation-and, in particular, the occurrence of sibling rivalry-has rarely been investigated in plants. In this article, we develop a new method for analyzing the resource-allocation traits of the different actors (maternal sporophyte and half-sibs) using data obtained from a large-scale diallel cross experiment in maize involving mixed hand pollination and color markers to assess seed weight of known paternity. We found strong evidence for the occurrence of sibling rivalry: resources invested in an ear were allocated competitively, and offspring with different paternal genotypes aggressively competed for these resources, entailing a measurable direct cost to the mother. We also show how resource allocation can be described for each genotype by two maternal traits (source effect, average sink responsiveness) and two offspring traits (ability to attract maternal resources, competitive ability toward siblings). We will discuss how these findings help to understand how genetic conflicts shape resource-allocation traits in angiosperms.

A NEW HYPOTHESIS FOR THE EVOLUTION OF OVERPRODUCTION OF OVULES: AN ADVANTAGE OF SELECTIVE ABORTION FOR FEMALES NOT ASSOCIATED WITH VARIATION IN GENETIC QUALITY OF THE RESULTING SEEDS

A new hypothesis for the evolution of overproduction of ovules within flowers is proposed: overproduction is a counter-strategy of female seed production in the conflict with males and/or offspring. It is advantageous for females to produce a uniform size of seeds, whereas it is advantageous for fertilized ovules to absorb more resources than this size. If there is a variance in resource absorption ability among fertilized ovules, nonuniform seeds are produced. Then, by overproducing ovules, females should select fertilized ovules with similar resource absorption rates, resulting in seeds of uniform size. A model analysis confirmed that this hypothesis works. In the model, the fertilized ovules of a plant consist of two genotypes that differ in resource absorption rate. I found that overproduction of ovules and selective abortion is advantageous if the difference in the resource absorption rates of the genotypes is large. The new hypothesis is different from the selective abortion hypothesis in that selecting ovules is advantageous even if there are no differences in the genetic quality of resulting seeds.

The Ecology and Physiology of Viviparous and Recalcitrant Seeds

▪ Abstract  Understanding seed physiology is central to reconstructing how angiosperms have evolved, to characterizing dormancy and germination regimes shared by suites of species, and to devising sound strategies for seed bank conservation, agriculture, and forestry. While species with dormant seeds have received the lion's share of attention, hundreds of plant species exhibit no seed dormancy and germinate either viviparously on the parent plant or shortly after release. Embryos of these recalcitrant and viviparous species cannot tolerate the maturation drying that is usually prerequisite to dormancy; such desiccation intolerance creates challenges for storing and preserving such embryos. I review the physiology, morphology, and ecology of these desiccation-intolerant, nondormant lineages. Differences in the production and function of plant hormones are implicated in the occurrence of recalcitrance and vivipary in plant families. Plant hormones are key regulators of seed physiology and simultaneously coordinate responses of the seedling and mature plant to their environment. Desiccation-intolerant embryos occur most commonly among species of wet or flooded environments and have evolved multiple times in disparate lineages. Natural selection in wetland environments simply may not eliminate these seed types or may select for changes in hormone physiology that simultaneously affect both maternal and embryonic tissues. Integrative data from ecological, genetic, and physiological studies are needed to elucidate evolutionary origins and maintenance of reproductive strategies in organisms.