User:Ptero-ADH/Isoetes

Reproduction

Description

Like the rest of the Lycopodiopsida class, Isoetes reproduces with spores. Among the lycophytes, both Isoetes and the Selaginellaceae (Spike Mosses) are heterosporous, while the remaining lycophyte family Lycopodiaceae (Club Mosses) is homosporous. As heterosporous plants, fertile Isoetes sporophytes produce megaspores and microspores, which develop in the megasporangia and microsporangia . The megasporangia occur within the innermost microphylls (single-veined leaves) of the plant while the microsporangia are found in the outermost microphylls. This pattern of development likely increases dispersal, which largely occurs in water. These spores then germinate and divide into mega- and micro- gametophytes. The microgametophytes have antheridia, which in turn produce sperm. The megagametophytes produce archegonia, which hold the egg cells. Outside of heterospory, a distinguishing feature of Isoetes (and other lycophytes) from ferns, is that their gametophytes grow inside the spores. This means that the gametophytes never leave the protection of the spore that disperses them, cracking the perispore (the outer layer of the spore) just enough to allow the passage of gametes. This is fundamentally different from ferns, where the gametophyte is a photosynthetic plant exposed to the elements of its environment. However, containment creates a separate problem for Isoetes, which is that the gametophytes have no way to acquire energy on their own. Isoetes sporophytes solve this problem by provisioning starches and other nutrients to the spores as an energy reserve for the eventual gametophytes. Although not a homologous process, this provisioning is analogous to other modes of offspring resource investment in seed-plants.

Reproductive cycle of Isoetes. The diploid sporophyte (A) produces microsporangia and megasporangia, which are located at the leaf bases. A cross section of the plant (B) shows that the megasporangia are located more towards the outer leaves (2) and the microsporangia are concentrated in the center (1). Via meiosis, the sporangia produce haploid spores (C). The megasporangia produce megaspores (3) which become female gametophytes and the microsporangia produce microspores (4) which become male gametophytes. The gametophytes germinate inside the spore, cracking the outer layer known as the perispore (5) as they grow via mitosis to expose the reproductive organs (6). Sperm from the male gametophytes locate the archegonia neck cells on the female gametophyte (6) and swim down to fertilize the egg. A diploid embryo is formed and a young sporophyte (D) is rapidly created through mitosis, eventually growing into another adult sporophyte.

Ecology

Like all plants, Isoetes undergoes an alternation of generations between a diploid sporophyte stage and a sexual haploid gametophyte stage. Throughout the evolutionary history of plants, this fundamental feature has been maintained, but the dominance of one stage over the other has shifted over time. The development of vascular tissue and subsequent diversification of plants coincides with the increased dominance of the sporophyte and reduction of the gametophyte. Isoetes, as members of the lycopods, are part of the earliest extant lineage that reflect this shift to a sporophyte dominant lifecycle. This fact cannot be overlooked because Isoetes evolved from earlier lycopods and share a common ancestor with Lepidodendrons that had a tree-like growth habit. Thus, the reproductive mechanisms of Isoetes are evolutionarily derived from large terrestrial plants, but tangled up in the necessities of aquatic reproduction.

Precious few studies have addressed reproductive ecology in Isoetes. While many of the pioneering studies of the late 1800's and early 1900's referenced above succeeded in describing the physical mechanisms of Isoetes reproduction, comparatively few studies have investigated ecologically relevant questions. For example, no study has undertaken a comparison of the importance of resource provisioning in the megaspore. Our understanding of other lineages would suggest that such a mechanism would have major consequences for fitness, yet this remains unexplored in Isoetes. The few studies on mating systems have suggested both obligate outcrossing and mixed mating systems occur, sometimes in the same species. Clearly more work is needed to understand the extent of variation in mating system within and between species. It is known that their reproductive ecology is highly tied to a water adapted lifecycle,