Flower and Plant Reproduction Before the Arrival of Bees: An Overview of Self-Fertilization and Cross-Fertilization

The world of flowers and plants has evolved dramatically over time, and one significant change in the reproductive strategies of plants occurred with the advent of bees as pollinators. However, before bees existed, plants had to rely on alternative methods of reproduction, primarily self-fertilization and cross-fertilization. This article delves into the mechanisms and dynamics of these reproductive processes, exploring how they have influenced the evolution of plants.

Self-Fertilization: The Process and Methods

Self-fertilization is a reproductive method where pollen from a single flower or plant fertilizes its own ovule. This process can be further divided into two primary types: autogamy and geitonogamy. In autogamy, the pollen is transferred within the same flower, while in geitonogamy, the pollen travels to a neighboring flower on the same plant (or in some cases, from a microsporangium to an ovule within a single monoecious gymnosperm).

Plants have developed various mechanisms to ensure autogamy. For instance, some flowers do not open, a phenomenon known as cleistogamy, or their stamens (the male reproductive structures) move to come into contact with the stigma (the female receptive part). The usage of terms like "selfing" is sometimes confused with self-fertilization, but it can also refer to other forms of selfing such as self-sterility or self-union.

Benefits and Challenges of Self-Fertilization

While few plants rely exclusively on self-fertilization, it has certain advantages and disadvantages. Advantages include:

Preserving genetic traits when pollinators are absent Enhancing reproductive success in isolated or sparsely pollinated regions (such as high altitudes or cold environments) Reducing energy expenditure on the production of attractants for pollinators Reducing dust wastage by avoiding transfers between flowers

However, self-fertilization can also limit genetic diversity and potentially lead to inbreeding depression. This diminishes the plant's ability to adapt to changing environments, making them more susceptible to diseases and less resilient in general.

Cross-Fertilization: Releasing Diversity

In contrast to self-fertilization, cross-fertilization involves the transfer of pollen from one flower to another. This method is facilitated by wind, water, and various animals, most notably insects such as bees.

Most flowering plants (80%) are bisexual, meaning they contain both male and female reproductive structures in the same flower. Approximately 5% of plant species are monoecious, meaning they possess both male and female structures on the same plant, while the remaining 15% are dioecious, meaning plants are either male or female.

Examples of plants that can self-fertilize include many species of orchids, peas, sunflowers, and tridax. Additionally, some plants like dandelions can perform both self-fertilization and cross-fertilization. The benefits of self-fertilization include:

Preservation of a suitable genotype in a specific environment Reduced dependence on pollinating agents, allowing reproduction when external vectors are absent Improved reproductive success when flowers are few or widely spaced

Despite these advantages, self-fertilization can also result in reduced genetic diversity and potential inbreeding depression. Many plants have evolved mechanisms to avoid self-fertilization, such as producing both self-pollinating and cross-fertilizing flowers.

Adaptive Evolution and Hybridization

The transition from outcrossing to self-fertilization is a common evolutionary process in plants. Around 10-15% of flowering plants are predominantly self-fertilizing. Examples include the shoe orchid (Paphiopedilum parishii), which secretes its pollen in a fluid state to self-fertilize, and the tree-orchid (Holcoglossum amesianum), which achieves self-fertilization by rotating its anther.

There are also mixed mating systems in some plant species, where each plant produces both self-pollinating and cross-fertilizing flowers. For instance, Bulbophyllum bicoloratum uses a rostellum to transfer pollen, and Caulokaempferia coenobialis uses a slippery emulsion to transfer pollen.

One species, Capsella rubella (Red Shepherd's Purse), has been found to have become self-fertile over 50,000 years, indicating that self-fertilization can be a long-lasting and stable evolutionary strategy for certain plants.

The maintenance of meiosis in self-fertilizing plants like Arabidopsis thaliana is an intriguing question. While self-fertilization reduces genetic diversity, meiosis can fix DNA damage, offering an adaptive advantage over asexual processes.