SPOROPHYTE: ORIGINS AND EXAMPLES - BIOLOGY - 2024

The sporophyte is the diploid multicellular stage in the life cycle of a plant or alga. It originates from the zygote produced when a haploid egg is fertilized by a haploid sperm, and therefore each sporophyte cell has a double set of chromosomes, one from each parent.

Land plants, and almost all multicellular algae, have life cycles where a multicellular diploid sporophyte phase alternates with a multicellular haploid gametophyte phase.

By Paul Garais, via Wikimedia Commons

Plants with seeds (gymnosperms) and flowering plants (angiosperms) have a more prominent sporophyte phase than the gametophyte and constitute green plants with roots, stem, leaves, and cones or flowers.

In flowering plants, gametophytes are small and supplanted by germinated pollen and the embryo sac.

The sporophyte produces spores (hence its name) by meiosis, which is a process known as "reduction division" that halves the number of chromosomes in each spore stem cell. The resulting meiospores (spores originating from meiosis) develop into a gametophyte.

The resulting spores and gametophyte are haploid, this means that they only have one set of chromosomes. The mature gametophyte will produce male or female gametes (or both) by mitosis.

The union of the male and female gametes will produce a diploid zygote that will develop into a new sporophyte. This cycle is called alternation of generations or alternation of phases.

Origin of the sporophyte

The origin of the sporophyte in terrestrial plants (embryos) represents a fundamental stage in evolutionary development. All organisms, except prokaryotes, undergo regular sexual reproduction that involves a regular alternation between meiosis and fertilization, expressing two alternate generations.

To try to explain the origin of alternate generations, there are two theories: the antithetical and the homologous. Based on evidence of the possible ancestors of land plants, the antithetical theory is accepted as more reasonable.

However, there are certain trade-offs with respect to the evolutionary process of bryophyte algae and the transition period of terrestrial plants to pteridophytes. These two major changes are best analyzed using neo-Darwinian theory and other evolutionary genetic processes as a reference.

The term terminal meiosis is also used, as this process occurs at the end of the life cycle of this cell line. These organisms are made up of diploid cells and haploid cells are represented by gametes.

In conclusion, the sporophyte does not form gametes but haploid spores by meiosis. These spores divide by mitosis and become gametophytes, which produce gametes directly.

Sporophytes in land plants

In these plant species, the life cycle is formed by an alternation of generations: from diploid sporophyte to haploid gametophyte. When the male gamete and the female gamete unite and fertilization occurs, a diploid cell called the zygote is created, which regenerates the generation of sporophytes.

In this way, the life cycle of the terrestrial plant is diplo-haplonic, with intermediate or spore meiosis. All land plants, with the exception of bryophytes and pteridophytes, are heterosporous specimens, which means that the sporophyte gives rise to two different types of sporangia (megasporangia and microsporangia).

The megasporangia give rise to the macrospores, and the microsporangia give rise to the microspores. These cells will develop into female and male gametophytes respectively.

The shape of the gametophyte and the sporophyte, as well as their degree of development, are different. This is what is known as alternate heteromorphic generations.

Sporophytes in bryophyte plants (algae)

The group of bryophytes, where mosses and liverworts are found, present a dominant gametophyte phase in which the adult sporophyte needs nutrition.

The embryonic sporophyte evolves by cell division of the zygote in the female sexual organ or archegonium, and in its early development, it is fed by the gametophyte. By having this embryonic characteristic in the life cycle, (common to all terrestrial plants), this group is given the name of embryophytes.

In the case of algae, there are generations of dominant gametophytes, in some species gametophytes and sporophytes are morphologically similar (isomorphic). In horsetail plants, ferns, gymnosperms, and angiosperms that have survived to this day, an independent sporophyte is the dominant form.

Evolution of bryophytes

The first terrestrial plants had sporophytes that produced identical spores (isospores or homospores). The ancestors of gymnosperms perfected complex heterosporic life cycles in which male and female gametophyte-producing spores were of different sizes.

Female megaspores tended to be larger and less numerous than male microspores.

In the Devonian period, some groups of plants independently evolved heterosporia, and later endosporia, in which gametophytes are minimally transformed within the wall of the spore.

In exosporic plants, among which are modern ferns, gametophytes come out of the spore, breaking the wall of the spore, and develop outside.

In endosporic plants, megagametophytes evolve within the sporangium to produce a very small multicellular female gametophyte that has female sex organs (archegonia).

The oocytes are fertilized in the archegonia with free-moving flagellated sperm, produced by miniaturized male gametophytes in the form of pre-pollen. The resulting egg or zygote was transformed into the new generation of sporophytes.

At the same time, the single large meiospore or megaspore contained in the modified sporangium of the original sporophyte is preserved within the pre-ovule. The evolution of heterosporia and endosporia are considered as some of the first steps in the evolution of the seeds that today's gymnosperms and angiosperms produce.

Bryophytes today

Throughout 475 million years, terrestrial plants have been perfecting and applying these evolutionary procedures. The 300,000 species of plants that exist today have a complex life cycle that alternates sporophytes (spore-producing organisms) and gametophytes (gamete-producing organisms).

In non-vascular plants, that is, they do not have a stem or root (green algae, mosses and liverworts), the structure visible to the naked eye is the gametophyte.

Unlike vascular plants like ferns and seed plants it has sporophytes. The sporophyte of a non-vascular plant generates haploid unicellular spores, and as a product of meiosis the sporangium.

Throughout the natural history of the earth, each species of plant manages to preserve independent development mechanisms in relation to embryonic processes and the anatomy of the species. According to biologists, this information is essential to try to understand the evolutionary origins of the alternation of generations.

1. Bennici, A. (2008). Origin and early evolution of land plants: problems and considerations. Communicative & integrative biology, 212-218.
2. Campbell, NA and Reece, JB (2007). Biology. Madrid: Editorial Médica Panamericana.
3. Friedman, W. (2013). One Genome, Two Ontogenies. Science, 1045-1046.
4. Gilbert, S. (2005). Developmental biology. Buenos Aires: Editorial Médica Panamericana.
5. Sadava, DE, Purves, WH. (2009). Life: The Science of Biology. Buenos Aires: Editorial Médica Panamericana.