Selective forces in the emergence of the seed habit

Abstract

I he evolution of'the seed is one of the major events in the history of land plants. In this paper, we consider the suite of characters that dcfinr the seed habit, and discuss the probable selective prrssures that produced each rharacter. Our major conclusion is that most rharacters are a direct consrquence of the origin of heterospory and of natural selection for propagules with larger lbod reserves.Seeds are traditionally defined by the possession of integuments. Howevcr, some heterosporous ptrridophytcs possess integumcnt-like structurcs. 'I'hrreforc, integuments cannot explain thr evolutionary success of scrd plants. Rather, we believe that the decisive character-in this succcss is related to pollination. Sred plants difkr Gom other heterosporous lineagcs i n the capturr of microspores beCore diqprrsal of the 'mrgaspore'. Modern gymnosperms all possess mrchanisms whcrrby the maternal sporophyte withholds resources from potential propagulcs that have not bccn pollinated and/or fcrtilizrd. 'l'his represents an inrrease in cfficirncy over pteridophytir reproduction. Wind-pollination means the propagule is vulncrablr to pathogens that mimic pollen, and pathogen pressures may have contributcd LO some seed charactcrs.

Novelties of the flowering plant pollen tube underlie diversification of a key life history stage

“…A host of key innovations have been proposed as correlates of the tremendous diversity and ecological success of angiosperms, and virtually all involve structural or life history traits of the sporophyte (1,5,11,(46)(47)(48)(49)(50)(51)(52)(53). However, based on the data presented here, male gametophyte (pollen tube) innovations that evolved early in angiosperm history were ''developmental enablers'' (54) for some of the most important reproductive traits of flowering plant sporophytes.…”

“…Virtually all of their most defining features, including the flower, closed carpel, highly reduced male and female gametophytes, double fertilization, sexually formed polyploid endosperm, and an exceptionally short pollination-tofertilization interval (progamic phase), are thought to have evolved under selection for a faster reproductive cycle (1)(2)(3)(4)(5)(6). To understand what changes may have contributed to speeding the progamic phase, I undertook a series of comparative analyses of the interacting ontogenies that determine fertilization timing in seed plants.…”

“…1). Because angiosperms generally have much shorter fertilization intervals, it has long been supposed that shortening of the fertilization process accompanied the origin of angiosperms (1)(2)(3)(4)(5). But two alternative hypotheses have also been proposed: the short fertilization interval may have evolved earlier in history, before the origin of angiosperms (6), or later, in one or more derived angiosperm lineages (7).…”

Sexual Conflict Arising from Extrapair Matings in Birds

“…First, sexual conflict over reproductive investment is ubiquitous in all systems in which lifetime monogamy does not occur (Fig. 3)-even without polyandrous matings, traits in males that enable them to extract levels of female parental investment above the female's optimum can be favored, including ejaculate components that manipulate females to increase their investment in the sire's brood (Chapman 2001;Wolfner 2002;Wedell et al 2006), and imprinted genes that, when inherited from the male, cause an embryo to increase the extent of resource extraction from its mother (Haig and Westoby 1989;Haig 2000). Second, polyandrous matings and multiple paternity (including EPP) in a given clutch or litter generate two additional episodes of sexual conflict that are absent when females mate with only one male in a given breeding seasonsexual conflict over both copulations and fertilization (Fig.…”

“…This is generally true because any deviation from lifetime sexual fidelity favors traits in both sexes that lead to increased investment in a given bout by the other sex. Mechanisms to achieve this increased investment include products in male ejaculates that alter female investment (Reinhardt et al 2009) or genomic imprinting that alters offspring physiology or behavior to extract more resources from the female (and possibly the pair male [Haig and Westoby 1989;Haig 2000]). However, when sexual conflict is driven by EPP, a reduction in parental investment by the within-pair male is favored, and Deviation from lifetime monogamous mating creates sexual conflict, but different mating patterns create different patterns of conflict.…”