Life histories, reproduction, and morphogenesis

Introduction The basic patterns of alternation of sporophyte and gametophyte must be regarded as a theme on which many variations are played (Fig. 2.1). Each generation may reproduce itself asexually, and sexual reproduction should be taken to include meiosporogenesis as well as gametogenesis and mating (Clayton 1988). Asexual reproduction allows an economical population increase but no genetic mixing, whereas sexual reproduction allows genetic mixing but is more costly because of the waste of gametes that fail to mate (Clayton 1981; Russell 1986; Santelices 1990). Most seaweeds use both means of reproduction, and, as Russell (1986) has noted, where there are isogametes, these can function equally as asexual swarmers. Vegetative reproduction by “multicellular propagules”, defined by Cecere et al . (2011) as “a vegetative, multicellular structure which detaches from the parent thallus and gives rise to a new individual”, is also common, for example Halimeda (Walters et al . 2002). However, their roles in species’ dispersal, and forming overwintering and resting “organs” that allow the survival of unfavorable environmental conditions, is unknown (Russell 1986; Cecere et al . 2011). Clonal seaweeds may spread by stolons and/or rhizomes, giving a significant competitive edge in the space race (sec. 1.2.3, 4.2.3). Some floating algal populations depend entirely on vegetative reproduction by fragmentation (sec. 3.3.7). Culture studies are critical in establishing the range of possible life histories that can occur. Sufficient variations have been discovered in the basic pattern – between and within species – that today’s generalizations must be viewed only as working hypotheses. New variations in what are considered to be well-known life cycles are regularly uncovered. For example, male gametophytes of Laminaria digitata can reproduce themselves via fragmentation (Destombe et al . 2011). Although a basic alternation of a sporophyte (typically diploid) and a gametophyte (typically haploid) is common among seaweeds various extras and shortcuts are known (Fig. 2.1). Indeed, a better generalization may be that almost any alternation is possible, and even no alternation at all. Moreover, the term “alternation” is a misnomer, in that it implies only two phases and a regular progression from one to the other; clearly that is not always the case (e.g. Scytosiphon , Fig. 2.2). Maggs (1988, p. 488) concluded that “life-history patterns seem to be more labile than morphological features, and the role of life-history variability in speciation, and in ecological success, should not be underestimated”.

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