Gametophyte and Sporophyte

Abstract

All land plants are believed to have had their origin in a unicellular green algal ancestor. The major innovations in the evolution of early land plants from their primitive ancestors were, not surprisingly, adaptations that allowed plants to reproduce in the absence of water. Besides a more sophisticated and differentiated multicellular anatomy, the invention of new reproductive structures and strategies were fundamental for the success of terrestrial colonization. One of the key innovations in the life history of land plants was the emergence of a haplo‚Äźdiplontic life cycle, in which an asexual diploid phase, the sporophyte, is predominant over a reduced and dependent sexual haploid phase, the gametophyte. Other evolutionary trends include a transition to a heterosporic life cycle, internalization of gametes within specialized gametophytes, development of a pollination mechanism independent of water for fertilization, and the invention of seeds, an efficient way of protecting and dispersing the developing embryos.

Key Concepts:

  • Paleobotany and phylogenetic analyses are important tools for inferring trends in the evolution and diversification of plants.

  • Land plant evolution involved innovations which allowed organisms to reproduce without a need for water.

  • Sporophytes are believed to have evolved from a haploid ancestral condition in which meiosis was delayed.

  • Multicellularity and sexual reproduction allowed diversification and adaptation to new environments during evolution.

  • Meiosis is essential in a sexually reproducing organism for maintenance of ploidy after fertilization.

  • Fertilization generates genetic diversity by combining chromosomes inherited from two distinct individuals.

  • Endospory evolved in the production of a seed.

  • Heteromorphism allowed evolution of sexual dimorphism.

  • Apomixis allows clonal propagation by seeds.

Keywords: meiosis; gamete; zygote; haploid; diploid; apomixes

Figure 1.

Life cycle of a green alga, Ulva. The sporophyte and gametophyte are multicellular and look identical (isomorphic). The gametophyte and sporophyte are both two cell‐layers thick. Cells on the sporophyte undergo meiosis and produce two types of spores, + and −. Each spore divides mitotically to give rise to a gametophyte (either + or −). The gametophyte produce (+) and (−) gametes. A (+) gamete fuses with a (−) gamete and forms the zygote. The zygote divides mitotically to give rise to the sporophyte.

Figure 2.

Life cycle of a brown alga, Fucus. The diploid individuals are large and branched. The tips of the branches are called receptacles. A cross‐section of the receptacle reveals its structure – meiosis occurs within the semi‐circular sacs called conceptacles. The oogonia form large eggs and the antheridia small sperm. These gametes are released and fertilization occurs. The fertilized eggs (zygotes) attach and grow into diploid individuals.

Figure 3.

Life cycle of a Bryophyte (moss). The moss sporophyte depends on the gametophyte and is not a free‐living independent plant. The spores form after meiosis and germinate to develop into male or female gametophytes. Moss sperm are produced from cells in the antheridia of the male gametophyte. The moss eggs are located at the base of the archegonia, on the female gametophyte. Each sperm has two flagella. Sperm swim to the eggs and after fertilization, the zygote develops into the sporophyte.

Figure 4.

Life cycle of a Pteridophyte (fern). Meiosis occurs within sporangia, located on the underside of the sporophyte leaf. After the spores are released they germinate, divide by mitosis and grow into simple heart‐shaped gametophytes. On the gametophyte, cells in the archegonium and antheridium form the eggs and sperm. The sperm swim to the egg and fertilize it, forming the zygote. The embryo develops into the sporophyte, still attached to the gametophyte.

Figure 5.

Life cycle of an angiosperm. The mature sporophyte produces flowers. Flowers contain male (stamens) and female (pistil) reproductive organs. In the pistil, the megaspore mother cells in the ovule undergo meiosis. Only one of the four products of meiosis survives and eventually develops into the eight‐celled female gametophyte, containing the egg cell, the central cell and a few other cells (antipodals and synergids). Within the anther, microspore mother cells undergo meiosis and form four haploid microspores. Unlike in the female, each microspore develops into a male gametophyte, or pollen grain, composed of only three cells. The pollen grain contains two sperm cells. Pollen grains land on the stigma surface of the pistil, grow a tube through the extracellular matrix of the style and arrive at the ovary by bursting through one of the synergid cells. The sperm cells cannot swim, but are delivered to the embryo sac by growth of the pollen tube. One sperm cell fuses with the central cell to form the primary endosperm cell and the other sperm cell fuses with the egg to form the zygote. The zygote develops into the sporophyte. Growth of the sporophyte is the result of mitotic divisions of cells in the root and shoot meristems.

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Friedman WE and Williams JH (2003) Modularity of the angiosperm female gametophyte and its bearing on the early evolution of endosperm in flowering plants. Evolution 57(2): 216–230.

Grossniklaus U (2001) From sexuality to apomixis: molecular and genetic approaches. In: Savidan Y, Carman JG and Dresselhaus T (eds) The Flowering of Apomixis: From Mechanisms to Genetic Engineering, pp. 168–211. Mexico, DF: CIMMYT, IRD, European Commission DG VI.

Li W and Ma H (2002) Gametophyte development. Current Biology 12(21): R718–R721.

The Plant Cell (2004) Special issue on the topic of plant reproduction. Plant Cell 16: S1–S245.

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Boavida, Leonor C, and McCormick, Sheila(Jan 2010) Gametophyte and Sporophyte. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002038.pub2]