Comparative studies on the male and female gametophyte development in three different Triticum species

Abstract

The in planta and in vitro development of the male and female gametophytes of three Triticum species (T. aestivum cv. Mv 15/6n/, T. araraticum/4n/ and T. monococcum/2n/) was traced from just after meiosis until the development of the mature pollen or embryo sac. The dynamics of differentiation in the male gametophytes was not influenced by the different ploidy levels up to the tricellular stage. The maturing process of differentiated haploid male gametophytes, resulting in the shedding of mature pollen from the anthers, proceeds most rapidly in the hexaploid species. The development of the initial microspore slowed in vitro, remaining in the uninucleate stage for a longer period, with a delay in first pollen mitosis compared to the control. There was no substantial difference between the three species in the dynamics of in vitro pollen development. In all cases the uninucleate microspore developed into a functional tricellular pollen grain within a week. However, there were differences in the lengths of the various developmental stages under in vitro and natural conditions. The seven-cellular female gametophyte developed far more rapidly than the male gametophyte, which means that the mitotic divisions leading to the differentiation of male and female gametophytes were not synchronised within the plants. In species with lower ploidy levels macrosporogenesis took longer than microsporogenesis, and the development of the seven-celled embryo sac was completed 2 days later than in hexaploid species. The differing ploidy levels also influenced the size of the embryo sac. The embryo sacs achieved their maximum dimensions 2–3 days prior to male gametophyte maturation. These observations raise several questions as to when gametophytes can be regarded as functionally mature, and which phase of gametophytic development is most suitable for cell manipulation.