Plant and Animal Reproductive Strategies: Lessons from Offspring Size and Number Tradeoffs

K. G. Srikanta Dani,Ullasa Kodandaramaiah

doi:10.3389/fevo.2017.00038

Abstract

The tradeoff between offspring size and number is ubiquitous and manifestly similar in plants and animals despite fundamental differences between the evolutionary histories of these two major life forms. Fecundity (offspring number) primarily affects parental fitness, while offspring size underpins the fitness of parents and offspring. We provide an overview of theoretical models dealing with offspring size and fitness relationships. We follow that with a detailed examination of life-history constraints and environmental effects on progeny size and number, separately in plants and animals. The emphasis is on seed plants, but we endeavour to also summarize information from distinct animal groups - insects, fishes, reptiles, birds, and mammals. Furthermore, we analyse genetic controls on progeny size and number in two model organisms - Arabidopsis and Drosophila. Despite the deep evolutionary divergence between plants and animals, we find four trends in reproductive strategy that are common to both lineages: (i) offspring size is generally less variable than offspring number, (ii) offspring size increases with increasing parent body size, (iii) maternal genes restrict offspring size and increase offspring numbers, while zygotic genes act to increase offspring size; such parent-offspring conflicts are enhanced when there is sibling rivalry, and (iv) variation in offspring size increases under sub-optimal (harsh) environmental conditions. The most salient difference between plants and animals is that the latter tend to produce larger (fewer) offspring under sub-optimal conditions while seed plants invest in smaller (many) seeds, suggesting that maternal genetic control over offspring size increases in plants but decreases in animals with parental care. The time is ripe for greater experimental exploration of genetic controls on reproductive allocation and parent-offspring conflicts in plants and animals under sub-optimal (harsh) environments.

Highlights

One of the most enduring research problems in evolutionary biology is understanding how organisms invest resources into reproduction, and the dynamic selective forces and tradeoffs that bear upon this
We summarize important life-history constraints on offspring size and number and explore evidence for parent-offspring conflict at the genetic level in model organisms (Arabidopsis and Drosophila)
Instead of providing a descriptive account of genes regulating seed size, we present an illustrative summary of seed size variation in Arabidopsis mutants (Figure 3; Table 1)

Summary

Introduction

One of the most enduring research problems in evolutionary biology is understanding how organisms invest resources into reproduction, and the dynamic selective forces and tradeoffs that bear upon this. The fact that work so far has looked at either plants or animals exclusively is understandable given the profound differences in ecological and life-history attributes of the two lineages, which diverged at least 1.5 billion years ago (Wang et al, 1999). Both plants and animals have diversified tremendously in their reproductive strategies, their fitness is inevitably determined by both quality and quantity of offspring, and is limited by resource availability. The cardinal theoretical construct for the offspring size-vs.-number tradeoff (Smith and Fretwell, 1974) was mostly agnostic with respect to the two taxonomic groups. There has been a substantial body of work dedicated to understanding variation in offspring size and number, but we still do not fully understand why and how this diversity evolves

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Conclusion