Selaginella--Heterospory

If you think for a moment about our path of evolution from algae through the ferns, you have observed the proliferation of adaptations that make organisms better-suited for terrestrial environments.

We have seen the development of a cuticle and the obvious compensatory development of functional stomata. We have seen the lignification of cells and proliferation of cell types for supporting a plant into the air against the gravity vector. We have observed several steps in the evolution of vascular systems to bring water to the aerial shoots. The spore walls were invested with sporopollenin to avoid desiccation during dispersal in wind.

You might notice that these changes were taking place primarily in the sporophyte or its products, the spores. This diploid plant has the fundamental advantage of sheltering silent recessive alleles providing a reservoir of genetic diversity for the evolution of the species. The gametophyte has remained a primitive relic and the weak link in the chain of the life cycle. It is not cutinized, not vascular, and dependent upon free-water for completing its critical function (syngamy). The gametophyte was dominant in bryophytes, but reduced in the ferns.

As we move into the study of Selaginella and other heterosporous ferns, the reduction of the gametophyte continues. Moreover, we will see the beginning an enshrouding process in evolution to shelter the much-reduced gametophyte within the better-protected sporophyte provisions.

The vegetative body of Selaginella is not particularly different from that of the other seedless vascular plants in the division Lycophyta. The stem is cutinized with an aerenchymatous cortex, and exarch, protostele (plectostele or haplostele). The leaves are microphylls with limited cutinized epidermis, mesophyll only a few cells thick and entirely spongy, and a single haplostelic vascular bundle. Typically the stem is dichotomously branched (primitive) and the spirally-arranged leaves are flattened dorso-ventrally into two morphs. The stem is held horizontally in most species and is suspended a few centimeters over the soil on rhizophores. These branch dichotomously but are positively gravitropic. Upon contact with soil (thigmomorphogenesis), they dichotomize into a fibrous root system with root hairs.

At the tips of the branches are found strobili. In various species the strobili may be held horizontally or may be vertically aligned. The microphylls in the strobilus are called sporophylls. Each sporophyll has a sporangium in its axil. The sporangium consists of a stalk and a sterile jacket of cells. Inside the sterile jacket is one or more sporocytes which ultimately divide by meiosis to produce spores. So far this description could be valid for any member of Lycophyta.

Closer examination of the strobili of Selaginella reveals that the sporangia are different in size and color. This is due to the presence of different contents. Sporangia near the base of a vertically-held strobilus or sporangia on the lower side of a horizontally-held strobilus are lumpy and lighter in color (typically yellow). These sporangia contain just four very large yellow spores. These four are the result of meiosis from a single sporocyte. The spores are called megaspores because of their large size. This makes the sporangium a megasporangium and the sporophyll a megasporophyll. The sporangia near the apex of a vertically-held strobilus or along the upper side of a horizontally-held strobilus are darker in color (typically orange) and are oval (not lumpy). These contain many very tiny orange spores, the products of meiosis of several sporocytes. These nearly-microscopic spores are called microspores. This makes their sporangium a microsporangium and the sporophyll a microsporophyll. It is not known mechanistically how the strobili achieve their dimorphic construction with respect to light and gravity vectors. There is more work for your generation of scientists to do!

Because Selaginella has both microspores and megaspores, the plant is called heterosporous. The other fern-allies we have studied were homosporous (having only one type/size of spore). This significance of this advance in evolution is important; sexual dimorphism is being expressed now much earlier in the life cycle--in the sporophyte! If the gametophyte is to be reduced and engulfed, then sexual expression must be shifted to the sporophyte. The organisms to appear next in our studies are ALL heterosporous.

As we follow these two kinds of spores we observe this reduction and engulfing of the gametophyte. The gametophyte that arises from the microspore is called a microgametophyte. The microgametophyte never leaves the wall of the microspore; it is endosporic. It is not photosynthetic but heterotrophic; its supply of nutrients is limited to what is contained in the original microspore. Its development is limited to the formation of an antheridium (sterile jacket) containing up to 32 sperm. Thus, the microgametophyte can be thought of as a male structure. The microspore wall and sterile jacket rupture in free water to release the sperm to swim to the egg. Since the microgametophyte is endosporic, the dispersal of male structures is a function of the microsporangium. Its sterile jacket resembles, in cellular details, the annulus of a fern, and operates in an analogous fashion. The endosporic microgametophytes are catapulted i nto the wind and the small structures can be dispersed widely this way. Note that the microgametophyte lacks rhizoids or sterile thallial cells. This reduction foreshadows further reductions in the seed-plants.

Meanwhile, the gametophyte that arises from the megaspore is called a megagametophyte. It too is endosporic and heterotrophic. Its large volume means that it can contain considerable reserves....usually oils. After cracking the megaspore wall slightly along the triradiate ridges, the megaspore divides into several sterile thallial cells that hold these nutrients, some rhizoid cells that protrude from the cracks, and a few archegonia with eggs. You will notice that the female gametophyte is less reduced than the male. In some species (arguably more primitive) the megaspore or endosporic megagametophyte is catapulted away from the sporophyte in the same way as microspores and endosporic microgametophytes. In other species (arguably more advanced) the sporangium opens, but the contents are not catapulted; in these, the microspores must blow into the open megasporangia and release sperm, or the sperm must swim into the megasporangium to get to the megagametophyte.

As we move into higher groups, the dispersal of endosporic microgametophytes will be a common feature called pollination. Likewise further engulfment of the megagametophyte will occur.

The Selaginella sperm must swim through the water-film up the rhizoids and into the cracked megaspore wall to reach the megagametophyte inside. The sperm swim down the open archegonium neck to arrive at the egg. The syngamy of egg and sperm results in a zygote. This developes into an embryo and ultimately an adult sporophyte.

The embryo will demonstrate root and shoot apices. Near its middle, a few embryonic cells will form a suspensor. These will elongate to push the embryo deeper into the surrounding megagametophyte tissue within the megaspore wall. At this point we have an embryo surrounded by storage tissue (megagametophyte in this case) and the whole contained in a coating (megaspore wall). This is almost a seed... almost because it lacks a multicellular seed coat. Nevertheless this formation is suggestive of seeds which we will observe in the next group, the gymnosperms.

The life history of Selaginella is interesting as it has three characteristics that are transitional between seedless vascular plants and the seed plants (gymnosperms and angiosperms):

The heterosporous condition of Selaginella (found in moist New England lawns) is shared by the spike worts (Isoetes...common to our pond-bottoms in New England), Salvinia (an introduced floating fern), and Marsilea (a tetrafoliate aquatic fern found on the edges of New England ponds).

In ancient times heterosporous seed ferns were a dominant form of vegetation on our planet, but are now extinct. A question to ask yourself might address why Selaginella and its kin are rather rare in the environment and why the seed ferns are extinct.