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In This Issue |
Piercey-Normore and DePriest test the often-assumed coevolution of algal and fungal partners in lichen associations by comparing evolutionary histories of species for coordinated speciation (cospeciation). Their data show that horizontal transfer of lichen algae is widespread in the Cladoniaceae (with some notable exceptions) and certain algal genotypes are more useful as food supplies to lichen fungi than others. In fact, like farmers, these fungi cultivate a few exceptional strains of their algal crop, which are dispersed and traded among large numbers of fungal partners. (see p. 1490)
Weevil predation promotes gynodioecy
Marshall and Ganders explore the maintenance of gynodioecy (females and hermaphrodites) in Sidalcea hendersonii (Malvaceae). Their field results indicate that the percentage of females found in natural populations is unusually high, and that a main factor perpetuating male sterility in these populations is that weevil larvae destroy significantly more seeds from hermaphroditic plants than female plants. This appears to be the first evidence that sex-biased seed predation may be a driving force in this mating system. (see p.1437)
No bottleneck for introduced orchid
Squirrell et al. add to the literature on the molecular divergence of alien species after introduction, in this case Epipactis helleborine (Orchidaceae), which was introduced from Europe into North America. The authors show that genetic diversity is not reduced in the North American relative to European populations as predicted by the population bottleneck hypothesis and document the lowest published interpopulation pollen:seed ratio, 1.43:1. This is very low compared with published estimates for other plant groups, but consistent with the high dispersibility of minute orchid seeds. (see p. 1409)
Watching the molecular clock
Sanderson and Doyle address a controversial issue, the age of the crown group of angiosperms, in a creative, complex, and exhaustive analysis. They show that age estimation is substantially more complicated than is generally recognized in the literature. Using the rbcL and 18S genes in a broad sampling of taxa, they demonstrate that confidence intervals on ages can be (1) very large, (2) extremely sensitive to type of data used (e.g., codon position or plant growth habit), but (3) surprisingly insensitive to important aspects of tree topology, such as seed-plant relationships. The age of angiosperms cannot be estimated by assuming a clocklike divergence of DNA sequences through time. (see p. 1499)
Phloem phylogeny
Turgeon et al. use a phylogenetic analysis for comparing phloem structure and loading mechanisms in an evolutionary context. Their new data on the ultrastructure of minor veins in Euphorbia and Celastrus were added to published data sets and mapped to a well-supported molecular tree. They conclude that extensive plasmodesmatal continuity between the mesophyll and minor vein companion cells is an ancestral condition in angiosperms. Although reduction in plasmodesmatal frequency is the general evolutionary trend, there have been occasional reversals. This study makes a bridge between what we know from classical systematic and morphological studies, new molecular data, and the growing number of tools for database analysis. (see p. 1331)
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