Sexual Reproduction

Asexual reproduction (vegetative or via mitospores) gives a quick and reliable result: daughter individuals are genetically identical to the parent (clones). In stable, predictable conditions, this is advantageous — a well-adapted genotype is replicated without the cost of finding a partner and complex meiotic processes (Graham et al., 2013). However, this uniformity has a downside.

If the environment changes (cooling, drought, appearance of a new pathogen), genetic uniformity becomes fatal. In asexual reproduction, all individuals share the same vulnerability. Sexual reproduction, on the contrary, creates unique combinations of genes each time. As biologists put it, this is a "natural experiment" (Mauseth, 2017). Even if most offspring turn out to be less fit than the parents, some may acquire new traits useful under the changed conditions. Thus, sexual reproduction is an insurance policy for the population against adverse environmental changes.

The evolutionary advantage of sexual reproduction is based on two mechanisms acting at the cellular level:

As a result, each gamete, and subsequently each zygote, carries a unique genetic "fingerprint". This creates material for natural selection and allows beneficial mutations to spread in a population, separating them from harmful genetic "ballast" (Barrett & Harder, 2017).

A key evolutionary acquisition of land plants is the alternation of generations (alternation of generations) with a dominant diploid phase (sporophyte) (Chapter 9; Mauseth, 2017; Becker et al., 2025). Why is this important? A diploid organism, carrying two sets of chromosomes, has backup copies of genes. If a harmful mutation occurs in one allele, the second, "healthy" allele can compensate. The haploid gametophyte, however, "sees" all mutations directly, making it more vulnerable.

During evolution from algae to angiosperms, a clear trend is observed:

This reduction of the gametophyte and the transition to a dominant diploid sporophyte allowed plants to colonize land. Protected by a spore coat (sporopollenin) and provided with a reserve of nutrients, the diploid embryo inside the seed became resistant to desiccation, temperature fluctuations, and could be dispersed over vast distances. Without sexual reproduction and meiosis, which restore ploidy and create diversity, this evolutionary leap would have been impossible (Chabert & Mallinger, 2025).

Understanding the evolutionary role of sexual reproduction has direct applied significance in agriculture. Most cultivated plants (fruit trees, berry bushes, many vegetables) are either completely self-sterile or partially self-sterile due to self-incompatibility mechanisms or early inbreeding depression (Chabert & Mallinger, 2025). In such conditions, cross-pollination (allogamy) is not just desirable but necessary for obtaining a yield.

The varietal diversity maintained by breeding is a direct consequence of sexual reproduction. It allows the development of hybrids with disease resistance, high yield, and improved taste qualities. However, as noted by Borghi et al. (2025), modern breeding sometimes weakens traits attractive to insect pollinators (e.g., reduces nectar production or aroma), which poses a threat to effective pollination. Therefore, understanding the evolutionary mechanisms of sexual reproduction helps agronomists and breeders develop sustainable production strategies, including the selection of pollinizer varieties and managing bee health.

Sporophyte (sporophyte, from sporo — spore and phyte — plant) is the diploid generation. This is what we usually see when referring to a fern, pine, apple tree, or wheat. The body of the sporophyte consists of cells with a diploid set of chromosomes (2_n_). The function of the sporophyte is to produce spores through meiosis. Specialized cells of the sporophyte (sporocytes, or spore mother cells) undergo meiosis, and each gives rise to four haploid spores (Yakovlev et al., 2005; Beck, 2010).

Gametophyte (gametophyte, from gameto — gamete and phyte — plant) is the haploid generation. Spores germinate and give rise to the gametophyte. Its cells already contain a haploid set of chromosomes (1_n_). On the gametophyte, in special organs (gametangia), gametes — spermatozoa or egg cells — are formed through mitosis (not meiosis!) (Serebryakova et al., 2006).

Thus, if in animals meiosis occurs directly during gamete formation, in plants meiosis occurs during spore formation. Gametes in plants are already formed on the gametophyte via mitosis (Graham et al., 2013).

Based on the type of spores produced, all higher plants are divided into two groups:

One of the main evolutionary directions in the plant kingdom is the gradual reduction (simplification and decrease in size) of the gametophyte and the strengthening of sporophyte dominance (Becker et al., 2025).

Why did evolution proceed towards strengthening the sporophyte? There are several explanations. The main one is that the diploid genotype is more resistant to harmful mutations. In the diploid sporophyte, most harmful recessive alleles do not manifest in the phenotype because they are "masked" by the normal dominant allele on the homologous chromosome. The haploid gametophyte, however, "sees" any mutation directly, and it can be lethal (Barrett & Harder, 2017; Becker et al., 2025). Therefore, the reduction of the gametophyte and the development of a powerful, protected sporophyte is a strategy to increase the reliability and fitness of plants in the diverse and often stressful conditions of terrestrial existence.

Furthermore, it was the dominance of the sporophyte that created the prerequisites for the emergence of the seed — a complex structure in which the diploid embryo (the future sporophyte) is surrounded by protective coats and provided with a reserve of nutrients (haploid in gymnosperms or triploid in angiosperms endosperm). The seed allowed plants to colonize the driest regions and became a crucial factor in the evolutionary success of flowering plants.

The entire life cycle of a higher plant can be represented as a sequential change of phases:

This cycle is called heteromorphic alternation of generations (because the sporophyte and gametophyte are not morphologically similar) (Mauseth, 2017; Strasburger, 1971).

The evolution of the sexual process in green plants proceeded from the fusion of identical cells to highly specialized oogamy (Yakovlev et al., 2005; Becker et al., 2025).

For flowering plants with bisexual flowers, the origin of the pollen (containing the male gametes) is of key importance.

In many bisexual plants, mechanisms have evolved that prevent self-pollination or make it sterile. These are self-incompatibility (self-incompatibility, SI) and self-sterility (self-sterility, SS).

Understanding these mechanisms is critically important for horticulture and seed production, as many fruit and berry crops are partially or completely self-sterile and require planting pollinizer plants (other varieties). For example, for apple, pear, cherry, and blueberry, it is necessary to plant at least two mutually pollinating varieties (Delaplane, 2023; cited in Chabert & Mallinger, 2025).

A special case of pollen influence on fruit and seed properties is xenia (xenia). This is the direct effect of genes from the paternal plant (pollen) on tissues that form after fertilization, such as the endosperm or seed coat. For example, in maize, when yellow grains are pollinated with pollen from a plant with purple grains, the endosperm may become purple, and in apples, pollination by different varieties affects the size, shape, and taste of the fruit (Denney, 1992; Liu, 2018). Xenia is not a self-incompatibility mechanism but shows that the choice of pollinator affects product quality even in self-fertile varieties (Chabert & Mallinger, 2025).