The Evolution of Embryo Size in Angiosperms and Other Seed Plants: Implications for the Evolution of Seed Dormancy

Abstract

Seed dormancy plays an important role in germination ecology and seed plant evolution. Morphological seed dormancy is caused by an underdeveloped embryo that must mature prior to germination. It has been suggested that the presence of an underdeveloped embryo is plesiomorphic among seed plants and that parallel directional change in embryo morphology has occurred separately in gymnosperms and in angiosperms. We test these hypotheses using original data on embryo morphology of key basal taxa, a published dataset, and the generalized least squares (GLS) method of ancestral character state reconstruction. Reconstructions for embryo to seed ratio (E:S) using family means for 179 families showed that E:S has increased between the ancestral angiosperm and almost all extant angiosperm taxa. Species in the rosid clade have particularly large embryos relative to the angiosperm ancestor. Results for the gymnosperms show a similar but smaller increase. There were no statistically significant differences in E:S between basal taxa and any derived group due to extremely large standard errors produced by GLS models. However, differences between reconstructed values for the angiosperm ancestor and more highly nested nodes are large and these results are robust to topological and branch-length manipulations. Our analysis supports the idea that the underdeveloped embryo is primitive among seed plants and that there has been a directional change in E:S within both angiosperms and gymnosperms. Our analysis suggests that dormancy enforced by an underdeveloped embryo is plesiomorphic among angiosperms and that nondormancy and other dormancy types probably evolved within the angiosperms. The shift in E:S was likely a heterochronic change, and has important implications for the life history of seed plants.

The role of temperature in post-dispersal embryo growth and dormancy break in seeds of Aconitum lycoctonum L.

“…Several authors have supported the hypothesis that an underdeveloped embryo is an ancestral character among seed plants (Forbis et al, 2002;Verdú , 2006). In accordance with this hypothesis, species of the Ranunculaceae considered basal in the Eudicots, in general have an underdeveloped embryo (Engell, 1995;Martin, 1946).…”

In situ observations of the basal angiosperm Amborella trichopoda reveal a long fruiting cycle overlapping two annual flowering periods

“…Morphological dormancy delays germination timing by the time the embryo needs to grow inside the seed before germination can take place (Baskin and Baskin 1998). This type of dormancy was found to occur in most of the ANITA grade (Forbis et al 2002) and may explain the delay of germination observed for A. trichopoda. Germination capacity seems associated with the color change of the exocarp as a germination rate of about 43 % was found for green/red fruits and over 80 % for red fruits.…”

Morfología, imbibición y germinación de semillas de Caesalpinia glabrata Kunth (Fabaceae) distribuida en un bosque seco tropical

“…The embryo is axillary, inverted foliate ( Figure 4A), i.e., it has large and thick cotyledons that enclose much of the radicle ( Figure 4B). Previous studies have argued that these embryos are more advanced and evolutionarily developed with respect to other types of plant embryos (Forbis, Floyd, & De Queiroz, 2002;Vanderlook, Verdú, & Honnay, 2012), being a beneficial trait in dry habitats (Vanderlook et al, 2012). A developed embryo would be ready to germinate during short periods of humidity in dry forests (Vivrette, 1995), after restrictions caused by the endosperm and coat are eliminated.…”

“…El embrion es axilar, foliado tipo invertido (Figura 4A), es decir presenta cotiledones grandes y gruesos que encierran gran parte de la radícula (Figura 4B). Estudios previos han argumentado que este tipo de embriones son más avanzados y desarrollados evolutivamente con respecto a otros tipos de embriones vegetales (Forbis, Floyd, & De Queiroz, 2002;Vanderlook, Verdú, & Honnay, 2012), siendo un rasgo beneficioso en hábitats secos (Vanderlook et al, 2012). Un embrión desarrollado estaría listo para germinar durante los periodos cortos de humedad en los bosques secos (Vivrette, 1995), luego que las restricciones provocadas por el endospermo y testa se elimimen.…”