MEIOSIS, BLADE DEVELOPMENT, AND SEX DETERMINATION IN PORPHYRA-PURPUREA (RHODOPHYTA)

The discovery in the early 1980s that meiosis occurs during germinatio n of conchospores of Porphyra yezoensis Ueda suggested that the sexual ly divided fronds of Porphyra purpurea (Roth) C. Agardh might similarl y originate from meiotic segregation of a Pair of sex-determining alle les during early sporeling development. After establishing conditions suitable for propagating P. purpurea in culture, observations on devel oping sporelings demonstrated that meiosis takes place during the firs t two divisions of the germinating conchospores. In the first division , the spore is split into an upper and lower cell. In the second, an a nticlinal division in the upper cell yields two daughter cells situate d one beside the other, and a periclinal division in the bottom cell g ives two cells arranged one above the other. Thus, during normal devel opment, the first four cells of the sporeling constitute a meiotic tet rad whose cells are arranged in a characteristic fashion. Stable color mutants of P. purpurea were isolated, genetically characterized, and used as genetic markers to follow the fate of individual cells of the tetrad during subsequent frond development. Nearly the entire blade of the mature thallus is derived from the two upper cells of the tetrad, with the two lower cells mostly giving rise to the rhizoidal holdfast region. Cell lineage boundaries laid down by the segregation of color alleles at meiosis corresponded perfectly with those later defined by sexual differentiation on the same fronds, strongly supporting the hy pothesis that sex determination in P. purpurea is controlled by allele s at a segregating chromosomal locus.