Formation of secondary pit connections by conjunctor cells in a coralline red alga

Abstract

ABSTRACT For more than eighty years, the accepted view of the origin of secondary pit connections in coralline red algae has been that a narrow channel of direct fusion forms between adjacent intercalary meristematic cells and a pit plug is subsequently deposited within the fusion channel; no cell or nuclear division is involved. Such a mechanism contrasts with the prevailing pattern of secondary pit connection formation among red algae in which a highly unequal, intercalary cell division produces a short-lived diminutive cell, the conjunctor cell, that fuses with an adjacent cell; the pit connection produced in the formation of the conjunctor cell is then shared by the two surviving cells. Study of Lithophyllum impressum by light and transmission electron microscopy revealed that asymmetrical lateral divisions of meristematic cells, as well as cells throughout the perithallus, produce tiny nucleated conjunctor cells. Thus, these findings indicate that secondary pit connection formation in this coralline alga does not occur by a direct mechanism nor are these connections generated only in the layer of intercalary meristematic cells as long believed.