On the Evolution of Complex Life Cycles in Plants: A Review and an Ecological Perspective

Abstract

Complex life cycles and alternation of generations are characteristic of many plants, a diploid sporophyte typically alternating with a haploid gametophyte. The prominence of each generation varies greatly among taxa. Purely phylogenetic or morphogenetic explanations of these differences are unsatisfying, as are those based solely on population fitness. Existing adaptational explanations seek selective advantages in diploidy and in sexual reproduction, but these explanations leave much to be explained -i.e., the existence of asexuality and of haploid organisms. Much of the existing variation in life cycles can be explained by selection on reproductive rates to meet the ecological problems of dispersal, colonization, niche preemption and exploitation, and mate competition. Placement of complex life cycles of plants in an ecological framework will, I hope, encourage specific studies exploring possible adaptive aspects and limitations on the evolution of life cycles in plants with different life histories.

Ontogenetic Loss of Phenotypic Plasticity of Age at Metamorphosis in Tadpoles

“…The Wilbur-Collins model was developed to explain plasticity in amphibian metamorphosis, but it has been applied successfully to development in insects (Blakley 1981 ), plants (Willson 1981, Lacey 1986, and fish (Policansky 1983, Reznick 1990. Further evidence for a transition from flexible to fixed developmental timing comes from Reznick's (1990) study of plasticity in age and size at maturity in male guppies (Poecilia reticulata).…”

Life-cycle-generation-specific developmental processes are modified in theimmediate uprightmutant of the brown algaEctocarpus siliculosus

“…8 Explaining the stability of haploid-diploid life cycles has been particularly problematic because, depending on the niche of the organism, either the haploid or the diploid generation is expected to present certain advantages and theoretical models predict that this should lead to a dominance of one generation over the other under a wide range of conditions (Mable and Otto, 1998). It has been suggested, however, that if the two generations are adapted to different ecological niches, this could stabilise a haploid-diploid life cycle (Stebbins and Hill, 1980;Willson, 1981) and this suggestion is supported by more recent theoretical work (Hughes and Otto, 1999). It is possible that the difference in morphology between the wild-type sporophyte and gametophyte of Ectocarpus reflects an adaptation to different ecological niches, with the dense, more robust thallus of the sporophyte, particularly its prostrate base, being better adapted for persisting in less favourable conditions during most of the year and the more fragile gametophyte being short-lived and adapted for producing gametes over a period of a few weeks.…”

Susceptibility of spores of different ploidy levels from AntarcticGigartina skottsbergii(Gigartinales, Rhodophyta) to ultraviolet radiation

“…The higher rate of DNA damage repair in haploid tetraspores may prove advantageous in the sense that deleterious mutations are purged more rapidly from haploid populations compared with diploid populations, which tend to mask deleterious alleles (Orr & Otto 1994;Hughes & Otto 1999). The existence and potential ecological advantages of alternation of generation between haploid-diploidy is thought to be the exploitation of a broader range of ecological niches, especially in environments that vary over space and time (Willson 1981). The extreme Antarctic environment characterized by low sea and air temperatures, annual extremes in light regime, wind speeds, disturbance, and isolation (Peck et al 2006), as well as the naturally lower net springtime ozone levels (Fahey 2003) might have favored diploidy, driving a shift in phase dominance in the local population.…”