Haploid and Diploid Phases

Valero M, Richerd S, Perrot V, Destombe C (1992) Evolution of alternation of haploid and diploid phases in life cycles. Trends Ecol Evol 7 25-29... 

The existence of different morphotypes, two ploidy levels related to phase changes, and the ability of both haploid and diploid stages to divide mitotically (Kornmann 1955 Rousseau et al. 1994 Vaulot et al. 1994), support the existence of a haploid-diploid life cycle in P. globosa. In such life cycles, both haploid and diploid stages are related by sexual processes, meiosis and syngamy, and both are capable of mitotic division (Fig. 3 ... 

Figure 9.9 A schematic representation of the cell cycle of a budding wine yeast cell. Haploid and diploid cells reproduce asexually by multilateral budding during which each cell gives rise to a daughter cell made of entirely new cell surface material. Buds may arise at any point on the mother cell surface, but never again at the same site. Under optimal nutritional and cultural conditions Saccharomyces cerevisiae doubles its mass every 90 min. The cell division cycle consists of four phases Gl, S, G2 and M.

Williamson, exponential or early stationary cultures, cultivated under conditions that either result in severe catabolite repression or its release, differ in their fraction of mtDNA/nDNA by 30% or less. We have confirmed these observations for the two haploid and diploid strains just mentioned when growing exponentially on 5% glucose (repressed), as well as for a commercial (Fleischmahn) (aneuploid) strain growing exponentially on lactate (derepressed), 5% glucose (repressed), and on 1% glucose in exponential (repressed) and early stationary (derepressed) phase. 

It is presumably the ability to survive as a heterozygote, even with one or more highly deleterious mutations, that has led to the dominance of the diploid phase in higher plants and animals.76 However, to the biochemical geneticist organisms with a haploid phase offer experimental advantages because recessive mutants can be detected readily. 

In vertebrates and diploid yeasts, cells in Gi have a diploid number of chromosomes (2/i), one inherited from each parent. In haploid yeasts, cells In Gi have one of each chromosome (l/i), the haploid number. Rapidly replicating human cells progress through the full cell cycle In about 24 hours mitosis takes —30 minutes Gi, 9 hours the S phase, lO hours and G2, 4.5 hours. In contrast, the full cycle takes only —90 minutes In rapidly growing yeast cells. 

The structures and life cycle of angiosperms are described briefly on pp. 29-30. The alternating haploid (n, gametophytic) and diploid (2n, sporoph ic) phases of the life cycle are diagrammed in Fig. 

Many details are still obscure, and data are only available from a limited number of species, but it appears that most coccolithophores alternate between fully armored (heterococcolith-covered) diploid life stages and less-well-armoured (either holococcolith-covered or else naked) haploid phases. Both phases are capable of indefinite asexual reproduction, which is rather unusual among protists. That sexual reproduction also occurs fairly frequently is evidenced by the observation of significant genetic diversity within coccolithophore blooms. Bloom populations do not consist of just one clone... 

The life cycle of Ceratopteris consists of two independent autotrophic phases, a diploid sporophyte and a haploid gametophyte. The diploid vascular sporophyte phase can be readily cultured in the greenhouse and consists of plants with a short upright stem, associated roots and upright leaves ranging in length from 0.2 to... 

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