LATENT DIPLOID PARTHENOGENESIS AND PARTHENOTE CLEAVAGE IN EGG-EQUIVALENTS OF NORWAY SPRUCE

It is widely thought that the absence of asexuality in the gymnosperms results from difficulties in evolving asexual reproduction. These difficulties have now been overcome under artificially controlled conditions. Specialized cells from suspension cultures of early embryos are shown to combine aposporous diploid parthenogenesis with cleavage. These latent and plastic sequential ontogenetic pathways are traced from isolated embryonal tubes that become initiated as putative central cells and produce binucleate egg-equivalents. In the absence of fertilization or fusion, egg-equivalent nuclei undergo parthenogenesis by forming a neocytoplasm and a thin cell wall. These are released as numerous diploid parthenotic leader cells into the culture medium. Parthenote (early embryo) development from leader cells occurs with a variable free-nuclear stage characteristic of zygotic and somatic proembryos. Parthenote development comprises a cleavage process that reconstitutes new individuals to further multiply a clonal population. The multiplication of free nuclei in egg-equivalents is stimulated by colchicine and directly contributes, through a homokaryotic advantage, to increased numbers of parthenotes and to parthenoclonal variations. The division and multiplication of free nuclei represents an early and latent form of classical cleavage polyembryony in Norway spruce.