On the genetic control of cell cycles during morphogenesis in Ulva mutabilis

Abstract

Development of the wild type and two temperature-sensitive mutants of the multicellular green alga Ulva mutabilis is compared. The mutants develop normal phenotypes at 22°C and abnormal phenotypes at 15°C. Normal development starts by formation of a filament consisting of a row of cells. The growth rate, the generation times, and the cell length at division change in a coordinated manner according to the positions of the cells within the filament. In the mutant cs 2 transfer to 15°C inhibits all cytoplasmic divisions during early development. In the mutant cs 6 the first three divisions proceed normally. Then cytoplasmic division is blocked in the most distal cells, while the proximal cells continue to divide according to a branched pattern. In the cs 2 mutant cell determination seems to occur at 15°C, while the differentiation of the determined cells can only occur at 22°C. In the mutant cs 6 the cells are not determined at 15°C. The cs 6 + gene, as well as the previously described Slender-like genes, presumably has a short period of activity and is concerned with more fundamental epigenetic processes than the cs 2 +-gene and the previously described precocious-like genes, which seem to have more prolonged periods of activity.