Understanding plant reproductive diversity

Abstract

Flowering plants display spectacular floral diversity and a bewildering array of reproductive adaptations that promote mating, particularly outbreeding. A striking feature of this diversity is that related species often differ in pollination and mating systems, and intraspecific variation in sexual traits is not unusual, especially among herbaceous plants. This variation provides opportunities for evolutionary biologists to link micro-evolutionary processes to the macro-evolutionary patterns that are evident within lineages. Here, I provide some personal reflections on recent progress in our understanding of the ecology and evolution of plant reproductive diversity. I begin with a brief historical sketch of the major developments in this field and then focus on three of the most significant evolutionary transitions in the reproductive biology of flowering plants: the pathway from outcrossing to predominant self-fertilization, the origin of separate sexes (females and males) from hermaphroditism and the shift from animal pollination to wind pollination. For each evolutionary transition, I consider what we have discovered and some of the problems that still remain unsolved. I conclude by discussing how new approaches might influence future research in plant reproductive biology.

Figure 1.

The three major evolutionary transitions in plant reproductive systems discussed in this article—the evolution of selfing from outcrossing, dioecy from hermaphroditism and wind pollination from animal pollination. SI refers to self-incompatibility. Note that in some transitions to selfing and dioecy the immediate ancestors may be wind pollinated.

Figure 2.

Study systems discussed in the text that have been used for research on plant reproductive diversity. (a) A long homostylous flower of Turnera ulmifolia var. angustifolia (Turneraceae) from Panama. Plants with this morphology exhibit mixed mating. (b) (i) Outcrossing and (ii) selfing flowers of tristylous Eichhornia paniculata (Pontederiaceae) from Brazil and Jamaica, respectively. (c) (i) Female and (ii) male plants of dioecious Wurmbea dioica (Colchicaceae) from South Australia. Note the conspicuous flower size dimorphism, a common feature of dioecious species. (d) (i) Hermaphroditic, (ii) female and (iii) male inflorescences of Sagittaria latifolia (Alismataceae) from S. Ontario, Canada. The hermaphroditic inflorescence is from a monoecious population and has male flowers at the top of the inflorescence and female flowers at the bottom. The female and male inflorescences are from a dioecious population. (e) Inflorescence of a male plant of dioecious wind-pollinated Thalictrum dioicum (Ranunculaceae) from S. Ontario, Canada. This genus contains both animal- and wind-pollinated species. (f) Inflorescences of monoecious wind-pollinated Carex pedunculata (Cyperaceae) from S. Ontario, Canada. Inflorescences are protogynous with male flowers at the top and female flowers below. All photographs taken by the author.