The deep sea is a vast and essentially continuous environment with few obvious barriers to gene flow. How populations diverge and new species form in this remote ecosystem is poorly understood. Phylogeographical analyses have begun to provide some insight into evolutionary processes at bathyal depths (<3000 m), but much less is known about evolution in the more extensive abyssal regions (>3000 m). Here, we quantify geographical and bathymetric patterns of genetic variation (16S rRNA mitochondrial gene) in the protobranch bivalve Ledella ultima, which is one of the most abundant abyssal protobranchs in the Atlantic with a broad bathymetric and geographical distribution. We found virtually no genetic divergence within basins and only modest divergence among eight Atlantic basins. Levels of population divergence among basins were related to geographical distance and were greater in the South Atlantic than in the North Atlantic. Ocean-wide patterns of genetic variation indicate basin-wide divergence that exceeds what others have found for abyssal organisms, but considerably less than bathyal protobranchs across similar geographical scales. Populations on either side of the Mid-Atlantic Ridge in the North Atlantic differed, suggesting the Ridge might impede gene flow at abyssal depths. Our results indicate that abyssal populations might be quite large (cosmopolitan), exhibit only modest genetic structure and probably provide little potential for the formation of new species.

Comparative genomic studies among highly divergent species have been problematic because reduced gene similarities make orthologous gene pairs difficult to identify and because colinearity is expected to be low with greater time since divergence from the last common ancestor. Nevertheless, synteny between divergent taxa in several lineages has been detected over short chromosomal segments. We have examined the level of synteny between the model species Arabidopsis thaliana and species in the Compositae, one of the largest and most diverse plant families. While macrosyntenic patterns covering large segments of the chromosomes are not evident, significant levels of local synteny are detected at a fine scale covering segments of 1-Mb regions of A. thaliana and regions of ,5 cM in lettuce and sunflower. These syntenic patches are often not colinear, however, and form a network of regions that have likely evolved by duplications followed by differential gene loss.