(American Journal of Botany. ;91:0.)
© Botanical Society of America, Inc.
Miocene winged fruits from Ecuador
Burnham and Carranco uncovered the remains of several inter-mountain lake basins that date from the Miocene in the Andes Mountains of Ecuador. The lake basins slowly filled with fine sediments, volcanic ash, and remains of plants, insects, and fish. Burnham and Carranco describe a new species of winged fruit from the Anacardiaceae (poison ivy, mango family) that is easily placed in the modern genus Loxopterygium and provides additional evidence that seasonally dry tropical forest was present in the inter-mountain basins of South America as much as 8-10 million years ago. When the tree of life, reconstructed using molecular evidence, is constrained using fossil evidence like this, certain branches of the tree can be firmly established using absolute time and thus give insights into the rate of evolutionary change.
(see front cover and p. 1767)
Raghavan examined the formation of early-stage somatic embryos on zygotic embryos of a transgenic stock of Arabidopsis thaliana, with a cyclin 1 At:beta-glucuronidase (GUS) reporter gene construct, after growth in a medium containing the synthetic auxin, 2,4-dichlorophenoxyacetic acid. After a brief period of cell expansion, divisions were initiated in the procambial cells facing the adaxial side at the base of the cotyledons. Cell divisions spread to the entire length of the cotyledons to form a massive callus with early-stage embryos. Transfer of the callus to an auxin-free medium led to the regeneration of mature-stage somatic embryos. Contrary to previous investigations that assigned a major role for the shoot apical meristem and cells in the axils of the cotyledons in the origin of somatic embryos on cultured zygotic embryos of A. thaliana, somatic embryos arose exclusively of the callus of cotyledonary origin.
(see p. 1743)
Armbruster and Rogers studied pollen competition in angiosperms in the context of outcrossing to find that pollen competition often increases the frequency of siring by genetically superior sporophytic fathers. Data collected from self-pollinated flowers of Dalechampia scandens vines support an alternative hyothesis=mthat pollen competition reduces negative effects of inbreeding. Offspring produced by self-pollinations with more intense pollen competition had higher fitness components that those produced by selfing with less-intense pollen competition; thus, the intensity of pollen competition may sometimes be an adaptation to reduce the cost of inbreeding.
(see p. 1939)
In fragmented populations, plant-pollinator interactions may become disrupted and reproduction may be reduced because of insufficient pollination (pollen limitation). Such pollen limitation reduced both the number and quality of offspring in this study of populations of the endangered, long-lived Scorzonera humilis (Asteraceae) by Colling et al. Seed set also increased with the local density of S. humilis plants, an indication that the reduced plant density in fragmented populations may reduce plant reproductive success.
(see p. 1774)
