Isoetes

Like all land plants, Isoetes undergoes an alternation of generations between a diploid sporophyte stage and a sexual haploid gametophyte stage. However, the dominance of one stage over the other has shifted over time. The development of vascular tissue and subsequent diversification of land plants coincides with the increased dominance of the sporophyte and reduction of the gametophyte. Isoetes, as members of the Lycopodiopsida class, are part of the oldest extant lineage that reflect this shift to a sporophyte dominant lifecycle. In closely related lineages, such as the extinct Lepidodendron, spores were dispersed by the sporophyte through large collections of sporangia called strobili for wind-based spore dispersal.[11] However, Isoetes are small heterosporous semi-aquatic plants, with different reproductive needs and challenges than large tree-like land plants.

Like the rest of the Lycopodiopsida class, Isoetes reproduces with spores[12]. Among the lycophytes, both Isoetes and the Selaginellaceae (Spike Mosses) are heterosporous, while the remaining lycophyte family Lycopodiaceae (Club Mosses) is homosporous.[13] As heterosporous plants, fertile Isoetes sporophytes produce megaspores and microspores, which develop in the megasporangia and microsporangia.[14] These spores are highly ornate and are the primary way by which species are identified, although no one functional purpose of the intricate surface patterns is agreed upon.[15] The megasporangia occur within the outermost microphylls (single-veined leaves) of the plant while the microsporangia are found in the innermost microphylls.[16] This pattern of development is hypothesized to improve the dispersibility of the heavier megaspore[12]. These spores then germinate and divide into mega- and micro- gametophytes.[14][17][18] The microgametophytes have antheridia, which in turn produce sperm.[18] The megagametophytes have archegonia, which grow the egg cells.[18] Fertilization takes place when the motile sperm from a microgametophyte locates the archegonia of a megagametophyte and swims inside to fertilize the egg.

Outside of heterospory, a distinguishing feature of Isoetes (and other lycophytes) from ferns, is that their gametophytes grow inside the spores.[14][18][16] 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.[18][19] Although not a homologous process, this provisioning is somewhat analogous to other modes of offspring resource investment in seed-plants, such as fruits and seeds. The extent to which resources provisioned to the megaspore also support the growth of the new sporophyte is unknown in Isoetes.

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.

Spore dispersal occurs primarily in water (hydrochory) but may also occur via adherence to animals (zoochory) and as a result of ingestion (endozoochory).[12][20] These are among the reasons suggested for the ornamentations of the spore, with some authors demonstrating that certain patterns seem well-adapted for sticking to relevant animals like waterfowl.[20] Another critical element of dispersal is the observation that in some species of Isoetes, the outer coat of megaspores have pockets that trap microspores, a condition known as synaptospory.[20][21] Typically, heterospory means that colonization and long-dispersal are more difficult due to the fact that a single spore cannot grow a bisexual gametophyte and thus cannot establish a new population from a single spore as can happen in homosporous ferns.[22] Isoetes may mitigate this issue via microspores stuck to megaspores, greatly increasing the possibility of successful fertilization upon dispersal.[20][21]