Reduced Independence of the Male Gametophyte in Angiosperm Evolution

Abstract

Trinucleate pollen, in which the generative cell has already divided into two sperm cells prior to dehiscence, is considered to have developed polyphyletically from the binucleate type in the course of evolution (Brewbaker, 1957, 1959, 1967; Pandey, 1960). Approximately 30 per cent of the Angiosperm plant families shed their pollens in the trinucleate stage. Physiologically, trinucleate pollens often differ from binucleate ones in their incompatibility system and have reduced storage longevity and germination capacities in vitro (Brewbaker, 1957, 1959, 1967). Recently we described their higher rate of respiration and loss of vitality in 97 per cent relative humidity at 30 °C (Hoekstra and Bruinsma, 1975). The present communication shows that trinucleate Compositae pollens already have highly organized mitochondria at the time of anther dehiscence and rapidly form pollen tubes without protein synthesis. In contrast, binucleate pollens show various stages of mitochondrial development at the time of dispersal, connected with differences in the rates at which their tube growth starts; moreover protein synthesis occurs to a different extent during tube growth. The differences in respiration in humid air are illustrated with three binucleate and one typical trinucleate species (Table 1). The specific details of pollen collection, storage, humification and germination, as well as the procedure of isolation of mitochondria, are described elsewhere (Hoekstra and Bruinsma, in press). Respiration at 30 °C, expressed as oxygen uptake per mg dry pollen, was determined by gas chromatography. Mitochondrial oxygen uptake was followed with