Colonial development in Pandorina morum. II. Colony morphogenesis and formation of the extracellular matrix

The colonial green alga, Pandorina morum, resembles its unicellular relative Chlamydomonas in both intracellular architecture and the composition of the extracellular matrix. Despite these similarities, cell division in Pandorina leads to the formation of a colony instead of the 8 or 16 single cells produced by cell division in Chlamydomonas. To study colony formation, partially synchronized cultures of P. morum were sampled periodically and stained with ruthenium red for electron microscopy. The cells of the colony were found to be held together during development by medial and basal connections between cells; the basal connections include strands of chloroplast. Studies of cells removed from the parental matrix before division confirmed that the cytoplasmic connections are strong enough to maintain the colonial configuration. After the medial connections break, the cells of the plate of the developing colony swing outward and attain the nearly spherical colonial configuration; the basal connections are still present. After this inversion, the formation of the extracellular matrix begins, with the colonial boundary appearing first. Capsule and sheath then form on the outer and inner faces of the colonial boundary until the extracellular matrix is complete. The process is compared to previous observations of Volvox, and possible evolutionary implications are discussed.