Exploring the universe of the pollen flowers: classification, traits, and distribution

Abstract

Pollination is a fundamental process for the reproduction and evolution of flowering plants. The costs and benefits associated with pollination can influence natural selection and shape floral traits over time, resulting in an impressive variety of shapes, colors, and floral structures in different species. This interaction can lead to coevolution, where the traits of both groups are shaped over time, optimizing pollination efficiency. However, in some flowers, such as pollen flowers, these costs and benefits are paid in the same coin, in this case with the pollen grains. In their original definition, these flowers were called “pollen flowers” by the botanist Stefan Vogel (1978), in contrast to other flowers that also offer another resource such as nectar, oil, or resin. The main function of pollen is to allow the transfer of male gametes to the female reproductive organ of the flower. While some plants rely on wind or water for pollen dispersal, others rely on pollinating insects, birds, bats, or other animals. The initial classification of pollen flowers was introduced by Vogel (1978), who proposed three distinct floral types: Magnolian, Papaver, and Solanum. In our first chapter, we provide a comprehensive review of Vogel's (1978) remarkable and highly significant work in the area of Pollination Biology. Furthermore, we reassess and propose an updated classification system for pollen flowers. This revised classification incorporates a broader range of botanical families known to possess pollen flowers, as well as additional floral traits. Our approach takes into account the phylogeny of angiosperms and uses new tools and methodologies that were not available during Vogel's era. In the second chapter, we summarize in a functional and evolutionary context what is known about morphological diversity, color patterns, and stamen specialization within Melastomataceae. First, we cover information on anther morphology, including dehiscence types, presence or absence of pedoconnectives, appendages, oil-glands, trichomes and rostrum. Then, we discuss variability in stamen shape and length, and their potential functional role in reproduction. Finally, the general role of stamen traits in dividing the pollen load between plant reproduction and pollinator reward, as well as alternative strategies, is explored as an adaptive explanation for the stamen diversity in Melastomataceae. In the third chapter of our study, we focused on analyzing the geographic distribution of Melastomataceae species that comprises a remarkably diverse group of flowering plants that thrive in different regions of the world, with a particular concentration in the Neotropic. We investigated different flower sizes and degrees of stamens dimetrism and its correlation between abiotic factors such as temperature, precipitation, and altitude with the geographic distribution of these traits.