Tetrasporangia

Tetrasporangia are sporangia that produce four spores by meiosis. The term is used primarily in phycology to describe structures in the sporophyte of certain red algae (Rhodophyta) and, less commonly, in some green and brown algae. Each tetrasporangium contains four haploid spores, called tetraspores, which are released when the sporangial wall dehisces.

In many red algae, the tetrasporangial phase is part of the life cycle: the diploid sporophyte forms tetrasporangia on its thallus; meiosis yields four genetically distinct haploid tetraspores that germinate to form the haploid gametophyte generation. The arrangement and number of tetrasporangia, as well as the pattern of tetrads, can be diagnostic of certain taxa; in some species, tetraspores are produced in a cruciate (cross-shaped) pattern within the sporangium.

In contrast to spores produced by mitosis (isospores) or by polysporangiia, tetraspores ensure rapid generation of four propagules from a single meiosis event. The term derives from Greek tetra, four, and sporangion, sporangium.