Prototype quillwort (Isoetes prototypus) COSEWIC assessment and status report: chapter 6

Biology

Life cycle and reproduction

Little or nothing is known about the various life cycle stages of Isoetes prototypus, or their requirements. Figure 9 shows a generalized illustration of the life cycle of an Isoetes sp.

There are apparently no publications on the reproduction and life cycle of Isoetes prototypus, and according to Dr. W. Carl Taylor (pers. com. 2003), there is very little information on the reproduction and life cycle of Isoetes species in general. According to Caplen and Werth (2000), “information on the natural history of sexual reproduction in Isoetes is completely lacking”, but direct observations of sexual reproduction in this genus have been made in vitro.

Virtually nothing is known about the factors that affect the survival of Isoetes prototypus, the age structure and stability of populations, or its reproductive and mortality rates. This species has evergreen leaves and is thought to be perennial. However, the in situ observations that have been made on I. prototypus were during the summer and early autumn.

Cobb (1963) provides one of the most comprehensible accounts of the life cycle and reproduction of Isoetes sp. The familiar Isoetes plants with the typical “quillwort” morphology are sporophytes that produce two types of spores, large female spores called megaspores, and small male spores called microspores. These spores are borne within sporangia located in the concavities of the lower spoon-shaped portions of the leaves (usually mainly the middle and outer leaves). Although both types of spores are usually produced by the same plant, the megaspores and microspores typically occur in separate sporangia; however, in some species both types of spores may occur together in the same sporangium. Up to 100 or more megaspores may develop within a single sporangium, whereas the number of microspores per sporangium may be as high as several hundred thousand. The spores typically mature in the late summer and are released as the sporangial tissue decays or breaks apart.

Figure 9. Life cycle of Isoetes (from Stern 1985; used by permission, McGraw-Hill Education).

Figure 9. Life cycle of Isoetes (from Stern 1985; used by permission, McGraw-Hill Education).

Microspores and megaspores each give rise to their own gametophytes or prothalli. The male gametophyte usually produces four motile spermatozoids, which are elongate, tiny and bear four cilia, two at either end. The female gametophyte is round and multicellular, and bears eggs along its dorsal aspect. The eggs are located at the bases of small funnel-like structures (archegonia) that open when the eggs are ready for fertilization and close after fertilization has occurred. The young sporophytes develop directly from the fertilized eggs.

The reproduction of Isoetes species is thought to be almost entirely sexual, by virtue of their heterosporous nature. However, Caplen and Werth (2000) speculate that “the close proximity of megasporangia and microsporangia coupled with the gradual release of spores through decay of sporangial tissue .…. might allow for significant levels of intergametophytic (i.e., sporophytic) selfing analogous to self-pollination in seed plants.”

Although many Isoetes hybrids have recently been reported in the literature and there is preliminary evidence that I. prototypus may have been a progenitor of I. tuckermanii, I. acadiensis and I. lacustris (Taylor 2003, pers. com.), no hybrids involving Isoetes prototypus have yet been detected.

Herbivory

It is not known if this species is a food source for animals but grazing by Mallards has been noted for other species of Isoetes (Brunton and Britton 1999).

Physiology

Little is known about the physiologic requirements of I. prototypus, other than the habitat data compiled in this report. No comprehensive study on the ecology of this species has been undertaken.

Dispersal/migration

Populations of aquatic species of Isoetes often exist in isolation from one another since the waterways or bodies of water where they occur are not interconnected (Caplen and Werth 2000). The movement or dispersal of Isoetes propagules has evidently never been observed in the field, but could occur by water flow within rivers and streams, or by the transport of propagules via animals such as beavers, muskrats or waterfowl (Caplen and Werth 2000, Brunton and Britton 1999), or possibly by moose. For example, Mallards have been seen grazing on other Isoetes and Common Loons have been seen uprooting and possibly eating Isoetes (Brunton and Britton 1999). Brunton (pers. com. 2004) also suggests that “fish and turtles are at least as likely vehicles of disturbance and transport”. 

There is apparently very little information on the movement or dispersal of Isoetes prototypus. However, at many lakes where it has been found, it was first detected by the presence of uprooted plants floating along the edges of the lake or washed up on shore. Brunton and Britton (1993) speculate that Isoetes plants may be dislodged by motor boats, fishing lines, anchors, swimmers, bottom-feeding waterfowl, muskrats, wintering reptiles and amphibians, and/or spawning fish. D.L. Sabine (pers. com. 2004) suggested that moose may perhaps play a role in dislodging plants of I. prototypus since well developed moose trails extend as deep as 1.5 to 2.0 m. in some New Brunswick lakes where I. prototypus occurs (e.g., site 12). Brunton (pers. com. 2002) has also proposed that the formation of gas pockets and/or the seasonal inversion of water may be responsible for dislodging Isoetes plants. In Holland Lake, plants of I. prototypus were sometimes easily dislodged by water currents that were created by the use of flippers, and partially dislodged mats of plants were sometimes found near the edges of underwater springs. Some of the dislodged floating plants are missing their corms and roots, but often still have microspores and macrospores (Goltz, personal observation). It is possible that movement and dispersal of floating plants of I. prototypus by wind and water currents could result in the deposition of spores and the formation of colonies at new sites within the lakes where this species occurs.

Interspecific interactions

Isoetes prototypus appears to be confined to oligotrophic lakes, where it typically grows in soft flocculent sedimentary ooze.

The writers confirmed that I. prototypus may form extensive swards on its own, but often is intermixed with swards of other Isoetes species and hybrids, especially I. lacustris and I. x harveyi, and dense mats of Eriocaulon aquaticum.

No hybrids involving I. prototypus have yet been found, including during fieldwork conducted by the writers during the summers of 2003 and 2004. However, such hybrids would best be detected by performing chromosome counts, and very few live specimens were collected for this purpose. It is very likely that this species does hybridize, or at least did in the past, since DNA analysis done by Dr. W. Carl Taylor suggests that I. prototypus has been involved in the genome of I. lacustris, and possibly I. acadiensis and I. tuckermanii.

Adaptability

Nothing is known about how well Isoetes prototypus tolerates environmental degradation or adapts to disturbance or change. It generally grows at depths exceeding 1.5 m. in cool oligotrophic spring-fed lakes, so it is speculated that it could be negatively impacted by water pollution, eutrophication, decreased water levels and changes in water temperature.

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