Overview

Sexually reproducing organisms alternate between haploid and diploid phases during one sexual generation, which extends from haploid phase to haploid phase (for animals and plants, from egg to egg).

During a sexual cycle, diploid cells produce haploid cells by meiosis, and haploid cells and their nuclei produce diploid cells by syngamy (fusion). Sexual life cycles are broadly classified according to whether mitotic nuclear division cycles occur in the diploid phase, the haploid phase, or both. These three types are as follows:

• Haplontic – predominantly haploid; mitosis does not occur in the diploid phase

• Diplontic – predominantly diploid; mitosis does not occur in the haploid phase

• Haplodiplontic – mitosis occurs in both the haploid and diploid phases

Eukarya also reproduce asexually. The kinds of asexual reproduction are classified at the end of the chapter. Parthenogenesis, an asexual mode of reproduction that evolved from sexual reproduction, is given special attention.

Evolutionary Considerations

There are wags who never tire of asking children, “Which came first, the chicken or the egg?” While this nonsensical question has no answer, it leads to a valid evolutionary question, “What was the relationship between haploidy and diploidy in ancient eukarya?” It is reasonable to assume that mitosis evolved before meiosis, at the beginnings of eukarya, and that the earliest eukarya were monoploid. Perhaps monoploid cells fused from time to time, yielding diploid cells.