Effects of the number of embryos in a seed and seed mass on seedling survival and growth in polyembryonic Ophiopogon japonicus var. umbrosus (Asparagaceae)

Research on polyembryony suggests that the presence of multiple embryos in a seed confers an advantage for seedling survival. Because observations from embryo to seedling stages are lacking, however, the effect of the exact number of embryos on seedling survival is unclear. In this study, we evaluated the effect of seed embryo number on seedling survival and growth to determine the number of embryos in a seed that are advantageous for seedling survival in Ophiopogon japonicus (Thunb.) Ker Gawl. var. umbrosus Maxim., which is a taxon exhibiting cleavage polyembryony. We also investigated whether seed mass affects seed embryo number and seedling survival and growth. We found that the number of embryos in seeds of O. japonicus var. umbrosus was weakly dependent on seed mass. As the number of embryos increased, the number of seedlings surviving from seeds initially increased and then decreased; the greatest number of seedlings was produced from an intermediate number of embryos, with the number of embryos producing the greatest number of seedlings increasing with seed mass. The sum of individual seedling lengths increased with the number of seedlings. Our results indicate that an intermediate number of embryos may be advantageous in polyembryonic O. japonicus var. umbrosus.

Proembryo Development and Suspensor Elongation in Araucaria Juss

Development of the proembryo and suspensor was studied in three species of Araucaria– A cunninghamii, A. bidwillii and A. heterophylla. In all species there are six synchronous free nuclear mitoses followed by wall formation. The subsequent internal division gives rise to a typical U.S.E. conifer proembryo. although the E group includes the conspicuous symmetrical cap, and the U cells are very ephemeral. During all divisions the proembryo is situated in the central region of the archegonium. The cells of the S group subsequently elongate to form the functional suspensor. Cleavage polyembryony does not occur although simple polyembryony is common. The proembryo of Araucaria is considered to have diverged at an early stage of evolution of the conifer proembryo, but to have subsequently undergone considerable specialization in a direction unrelated to evolutionary trends recognizable among other conifers.

POLYEMBRYONY IN TOMATOES

Twenty-nine polyembryonic seeds were found among 26,000 tomato seeds. These gave rise to twin seedlings. Five of the polyembryonic seeds appeared as a "double" seed with two units joined at adjacent sides to form the seed. Five pairs of conjoined seedlings were found. In some pairs, one member was four or five times larger than its mate. All the mature twins appeared to be diploid. One true-breeding mature twin may have been a haploid whose chromosome complement was spontaneously doubled. Most of the twin seedlings were from parents which were heterozygous for the gene U (uniform green fruits). The U-genotypes of 32 such twins were determined from F3 segregation. The results indicate that most twins resulted from normal segregation and recombination. Therefore, simple polyembryony appears to be the most likely origin of most tomato twin seedlings. False polyembryony due to fusion of two ovules, and euploid polyembryony may account for a few twins. Sporophytic and cleavage polyembryony appear to be relatively unimportant.