The Diplohaplontic Life Cycle


Seed plants have a diplohaplontic life cycle which has two phases. The first is a dominant, concurrent sporophyte phase and the second is a brief gameto-phyte phase; both phases are multicellular so somatic growth follows meiosis and somatic growth follows the union of gametes or syngamy. But even so, there are variants within this basic plan. Some angiosperm species, like bamboo, flower only once at the end of life. Conifers, by contrast, are long-lived iteroparous species which can produce male and female gametophytes annually for hundreds or even thousands of years. Each year, the adult sporophyte gives rise to a new set of indeterminate meristems at the telescoping ends of the latest branch tips and some of these branch tips develop either male or female reproductive initials. Conifers thus have meristems which produce both vegetative and reproductive growth as opposed to the predetermined germline typical of the diplontic life cycle of vertebrates. This “moving interface” between vegetative and reproductive cell lineages has not been well-studied in conifers and it constitutes a fertile area for exploring adaptive response, mutational accumulation, DNA repair systems and regulatory cues.


Pollen Tube Apical Meristem Shoot Apex Female Gametophyte Somatic Growth 
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