Morphological and phenological studies on sexual reproduction of Preissia quadrata (Marchantiaceae, Marchantiophyta)

Abstract

Preissia quadrata (≡Marchantia quadrata) is a complex thalloid liverwort that shows unique sexual differentiation pattern. In this paper we summarized the morphology and developmental season of sexual reproductive branches and sporophytes of the plants growing in the alpine regions of Japan. The specialized sexual reproductive branches (gametangiophores) have an umbrella shape with a stalk. The upper part of the male gametangiophore (antheridiophore) was dish-shaped, while the upper part of the female gametangiophore (archegoniophore) was gently lobed into four. After the spring thaw, the archegoniophores that were fertilized the previous year and overwintered elongated its stalk and released spores. The antheridiophores that overwintered in an undifferentiated state also rapidly developed, elongated its stalk, and released sperms. At the same season, the archegoniophores were only about 1 mm in diameter and their stalks were still short, but they had differentiated archegonia, and fertilization occurred if there was an antheridiophore nearby. The fertilized archegoniophores overwintered with young sporophytes. Once a thallus had formed a reproductive branch, it stopped growing and then branched from its ventral tip to form a new thallus. This new thallus may develop a sexual branch of a different sex from those of the original thallus. In addition to typical archegoniophores and antheridiophores, which have reproductive organs of only one sex, androgynous reproductive branches were also found. Tiny sexual reproductive branch primordia, less than 1 mm in diameter, were observed almost throughout the year. These phenomena are also known in European and North American populations and indicate that the species is essentially monoicous (or autoicous). The sporophytes are located one (rarely two) at the base of each of the loosely divided lobes of archegoniophore. Unlike Marchantia species, the spore mother cells were polyplastidic with numerous plastids within the cells. The sporophyte was triple-protected by the calyptra, pseudoperianth, and involucre. If fertilization is not successful, the archegoniophore will not develop pseudoperianth, involucres, long stalks, and withered while remaining small.