2024-05-07 16:46:37
Sexuality is enormously diverse throughout the tree of life and plants are a good example of this. Those that have seeds, the spermatophytes, include gymnosperms (a group with about 800 species, among which pines, firs or cypresses are its best-known members) and angiosperms (about 300,000 species, including all plants that produce flowers showy and fruits; almost all those that support our diet such as cereals, legumes or fruit trees).
The strategies of spermatophytes to generate offspring are very varied. Sexuality and the “search for a partner” have been an unparalleled evolutionary driver for them because, being sessile organisms, it is more difficult for them. In their case, the male gametes, carried in the pollen that forms in the anthers, must fertilize the ovules, located at the base of the pistils.
Parts of a mature flower. Mariana Ruiz / Wikimedia Commons, CC BY
For this to happen, they must resort to pollination vectors, such as wind, water or certain animals that seek a reward in the flowers.
The sex of spermatophytes is determined by their chromosomes, various genes and their interactions, by epigenetic factors (which determine gene expression) and by the resulting hormones. They are molecular cascades, sometimes modulated by environmental factors, that vary between and within species, sometimes even within the same individuals.
The distribution of resources to each sex is variable: species with hermaphroditic flowers and dioecious ones (which have separate male and female individuals) are just the two extremes of a continuous gradient in which we find all possible combinations of sexuality.
Male, female and the opposite
Monoecious species have unisexual flowers, but both male and female flowers appear on all individuals. Likewise, there are species with different combinations of hermaphrodite, male and female flowers within each individual (the gynomonoecious, andromonoecious and androgynomonoecious species), and species with all possible combinations of hermaphrodite, male and female individuals (the gynodioecious, androdioecious, androgynodioecious species). , gynomonodioecious, andromonodioecious and monodioecious).
Furthermore, most dioecious species present some lability in their sexual expression, with individuals changing from male to female or vice versa at some point in their lives. In some cases, they are not isolated individuals but entire populations, subject to different conditions, which differ in their sexual system.
Strawberries (genus Fragaria) reproduce using stolons, so they are self-sufficient for the continuity of their species. Mark Hofstetter / Wikimedia Commons, CC BY
On the other hand, not all hermaphrodite plants are the same. While some take advantage of their bisexuality to self-fertilize without having to find a partner, others avoid it with different mechanisms. Each of these strategies has its pros and cons: avoiding self-fertilization favors genetic diversity, while favoring it ensures offspring in conditions where finding a partner may be difficult.
At one extreme, asexual plants, which reproduce by apomixis (they generate their own seeds without the need for fertilization) or vegetative multiplication (like strawberries with their runners), embody the maximum expression of the self-sufficiency strategy.
Hermaphrodites for all tastes
Among the hermaphrodites that rely on self-fertilization we find the cleistogamous species, which have flowers that self-fertilize without even opening, and the selfers, which have small, inconspicuous flowers without odors or nectar, since they do not require attracting any pollinator.
Apomixis is a very common mode of reproduction among angiosperms. The popular dandelion (Taraxacum officinale) is an apomictic species. Pöllö / Wikimedia Commons, CC BY
Here we even find the case of The very Incanwhich is capable of self-pollination with active movements, rubbing its anthers on its stigma as the flower begins to open.
Among the hermaphrodites that avoid self-fertilization, we find the self-incompatible species, the dichogamous and the heterostylous.
The former present genetic mechanisms that block pollen access to the ovule, something that sometimes also makes it reject the pollen of some of its congeners.
Aeonium wavy, an endemism of Gran Canaria, is a dichogamous species in which there is a temporal separation in the maturation of the sexes within the same flower or the same plant. Nadiatalent / Wikimedia Commons, CC BY
Dichogamous species have flowers that are first male and then female (the protrandic), or vice versa (the protogynous), or with half of the individuals in each population being protrandic and the other half protogynous (the heterodichogamous).
In the populations of heterostylous species there are also two different types of individuals – known as morphs – whose female and male sexual organs are arranged at different heights and in a reciprocal manner.
Thus, the two morphs are pollinated and fertilized by a precise pollen transfer in different parts of the pollinator’s body. This hypothesis, already proposed by Darwin, has recently been confirmed.
By observing correlations between floral traits, it was found that heterostyly evolves in flower lineages with a narrow floral tube and pollinators such as butterflies or moths, pieces that fit together like a puzzle so that pollen can be transferred precisely from one morph to another.
Spermatophytes are not the most diverse group of living beings, but they do account for most of the terrestrial biomass. Although they present many other reproductive strategies that we left in the pipeline, let this tongue twister serve to illustrate that sexual diversity is a natural and ubiquitous reality in the tree of life.
#fluid #sex #flowers