Life cycles

<jats:title>Abstract</jats:title> <jats:p>Alternations of morphologically distinct generations are frequent in the Haptophyta, but the existence and place of sexuality, if applicable, often remains unknown. In most Pavlovales a nonmotile stage exists as an alternative to flagellate cells and is assumed to respond to environmental conditions. Similarly palmelloid and motile stages are recorded in the Isochrysidales where the dominant stage is planktonic (lsochrysis) or benthic ( Chrysotila). In the Prymnesiales a complex cycle is suspected for Phaeocystis involving colonial and motile stages, although observations on the nuclear ploidy levels of the different phases are lacking. Except in Platychrysis where amoeboid cells produce swarmers, members of the Prymnesiaceae are flagellates in culture, with resting cysts recorded in Prymnesium. Two motile cell types, with different scales, recently described in Chrysochromulina polylepis cultures could possibly indicate sexuality in Chrysochromulina. Many Coccolithophorales have heteromorphic life histories. In coastal genera (Pleurochrysis, Hymenomonas, Ochrosphaera), diploid coccolith-bearing cells and haploid scale-covered forms alternate, with different patternings on the organic scales of both phases. Life histories combining hetero- and holococcolithophorids are noteworthy: increasing numbers of oceanic holococcolithophorids, previously considered as autonomous species, are now shown to be alternate phases in the life cycle of heterococcolithophorids, but, as often in the Prymnesiophyceae, information on the nuclear cytology is needed.</jats:p>