The recessive rga mutation is able to partially suppress phenotypic defects of the Arabidopsis gibberellin (GA) biosynthetic mutant ga1-3. Defects in stem elongation, flowering time, and leaf abaxial trichome initiation are suppressed by rga. This indicates that RGA is a negative regulator of the GA signal transduction pathway. We have identified 10 additional alleles of rga from a fast-neutron mutagenized ga1-3 population and used them to isolate the RGA gene by genomic subtraction. Our data suggest that RGA may be functioning as a transcriptional regulator. RGA was found to be a member of the VHIID regulatory family, which includes the radial root organizing gene SCARECROW and another GA signal transduction repressor, GAI. RGA and GAI proteins share a high degree of homology, but their N termini are more divergent. The presence of several structural features, including homopolymeric serine and threonine residues, a putative nuclear localization signal, leucine heptad repeats, and an LXXLL motif, indicates that the RGA protein may be a transcriptional regulator that represses the GA response. In support of the putative nuclear localization signal, we demonstrated that a transiently expressed green fluorescent protein-RGA fusion protein is localized to the nucleus in onion epidermal cells. Because the rga mutation abolished the high level of expression of the GA biosynthetic gene GA4 in the ga1-3 mutant background, we conclude that RGA may also play a role in controlling GA biosynthesis.

Structural maintenance of chromosomes (SMC) proteins function in chromosome condensation and several other aspects of DNA processing. They are large proteins characterized by an NH2-terminal nucleotide triphosphate (NTP)-binding domain, two long segments of coiled coil separated by a hinge, and a COOH-terminal domain. Here, we have visualized by EM the SMC protein from Bacillus subtilis (BsSMC) and MukB from Escherichia coli, which we argue is a divergent SMC protein. Both BsSMC and MukB show two thin rods with globular domains at the ends emerging from the hinge. The hinge appears to be quite flexible: the arms can open up to 180°, separating the terminal domains by 100 nm, or close to near 0°, bringing the terminal globular domains together.A surprising observation is that the ∼300–amino acid–long coiled coils are in an antiparallel arrangement. Known coiled coils are almost all parallel, and the longest antiparallel coiled coils known previously are 35–45 amino acids long. This antiparallel arrangement produces a symmetrical molecule with both an NH2- and a COOH-terminal domain at each end. The SMC molecule therefore has two complete and identical functional domains at the ends of the long arms. The bifunctional symmetry and a possible scissoring action at the hinge should provide unique biomechanical properties to the SMC proteins.

Recently, there has been much interest in detecting and measuring patterns of change in disparity. Although most studies have used one or two measures of disparity to quantify and characterize the occupation of morphospace, multiple measures may be necessary to fully detect changes in patterns of morphospace occupation. Also, the ability to detect morphological trends and occupation patterns within morphospace depends on using the appropriate measure(s) of disparity. In this study, seven measures were used to determine and characterize sensitivity to sample size of the data, number of morphological characters, percentage of missing data, and changes in morphospace occupation pattern. These consist of five distance measures—sum of univariate variances, total range, mean distance, principal coordinate analysis volume, average pairwise dissimilarity—and two non-distance measures—participation ratio and number of unique pairwise character combinations. Evaluation of each measure with respect to sensitivity to sample size, number of morphological characters, and percentage of missing data was accomplished by using both simulated and Ordovician crinoid data. For simulated data, each measure of disparity was evaluated for its response to changes of morphospace occupation pattern, and with respect to simulated random and nonrandom extinction events. Changes in disparity were also measured within the Crinoidea across the Permian extinction event.Although all measures vary in sensitivity with respect to species sample size, number of morphological characters, and percentage of missing data, the non-distance measures overall produce the lowest estimates of variance (in bootstrap analyses). The non-distance measures appear to be relatively insensitive to changes in morphospace occupation pattern. All measures, except average pairwise dissimilarity, detect changes in occupation pattern in simulated nonrandom extinction events, but all measures, except number of unique pairwise character combinations and principal coordinate analysis volume, are relatively insensitive to changes in pattern in simulated random extinction events. The distance measures report similar changes in disparity over the Permian extinction event, whereas the non-distance measures differ. This study suggests that each measure of disparity is designed for different purposes, and that by using a combination of techniques a clearer picture of disparity should emerge.

The recessive rga mutation is able to partially suppress phenotypic defects of the Arabidopsis gibberellin (GA) biosynthetic mutant ga1-3 . Defects in stem elongation, flowering time, and leaf abaxial trichome initiation are suppressed by rga . This indicates that RGA is a negative regulator of the GA signal transduction pathway. We have identified 10 additional alleles of rga from a fast-neutron mutagenized ga1-3 population and used them to isolate the RGA gene by genomic subtraction. Our data suggest that RGA may be functioning as a transcriptional regulator. RGA was found to be a member of the VHIID regulatory family, which includes the radial root organizing gene SCARECROW and another GA signal transduction repressor, GAI . RGA and GAI proteins share a high degree of homology, but their N termini are more divergent. The presence of several structural features, including homopolymeric serine and threonine residues, a putative nuclear localization signal, leucine heptad repeats, and an LXXLL motif, indicates that the RGA protein may be a transcriptional regulator that represses the GA response. In support of the putative nuclear localization signal, we demonstrated that a transiently expressed green fluorescent protein-RGA fusion protein is localized to the nucleus in onion epidermal cells. Because the rga mutation abolished the high level of expression of the GA biosynthetic gene GA4 in the ga1-3 mutant background, we conclude that RGA may also play a role in controlling GA biosynthesis.

It is widely believed that morphological constraints are responsible for the observed pattern of decreasing major morphological innovation in both the Metazoa and Metaphytes over geological time. This is readily seen as the decreasing trend of origination of higher taxa: phyla, classes, and orders. Currently, there are two competing evolutionary hypotheses that have been proposed to explain this phenomenon: (1) the empty ecospace hypothesis and (2) the developmental constraint hypothesis. To distinguish between hypotheses 1 and 2, the change of morphological innovation before and after several mass extinction events was measured in the Crinoidea and Blastozoa. Mass extinction intervals provided a means in which to remove ecospace limiting constraints and allow the developmental constraint hypothesis to be thoroughly tested. Within the Crinoidea, disparity was measured before and after three mass extinctions. Within the Blastozoa, disparity was measured before and after two mass extinctions. For each taxon, three suites of characters were analyzed: ecological, nonecological, or "developmental" and a combination of the two previous suites plus 50 additional characters. Four different measures of disparity were used to analyze each character suite. In the majority of the cases investigated, disparity rebounds to comparable levels or in some cases higher levels in both the Crinoidea and Blastozoa. The results indicate that developmental constraints are not responsible for the decrease in disparity throughout the geologic range of the taxa. The more likely scenario is that increasingly structured ecological guilds have made it much more difficult to allow large increases in disparity.

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Late Cretaceous marine reptiles-Marine reptiles were the dominant predators of the Jurassic and Cretaceous seas. Large, mobile, fully pelagic, predatory groups include the orders Ichthyosauria, Sauropterygia (plesiosaurids and pliosaurids), and the family Mosasauridae. Although the basic dental crown design is a simple cone, many complex morphologies have arisen, including modified designs for piercing, crushing, cutting, and tearing

The pattern of decreasing disparity has been observed in both the metazoans and metaphytes throughout the Phanerozoic. The pattern is manifest as a decreasing trend in the origination of higher taxa. Currently, two competing evolutionary hypotheses have been proposed to explain this phenomenon: the empty ecospace hypothesis and the developmental constraint hypothesis. To empirically distinguish between these hypotheses, the change in disparity before and after the end-Permian mass extinction event was measured in the articulated brachiopods. The assumption is that ecospace-limiting constraints are removed after mass extinctions revealing the effect of developmental constraints. For each taxon within the group, both continuous and discrete character sets were analyzed. Four different measures of disparity were used to analyze each character suite. Additionally, a separate analysis was performed on a subset of the articulated brachiopods, the rhynchonellids and terebratulids. In most cases investigated, disparity rebounded to comparable levels, with the rhynchonellids and terebratulids showing the largest increase in disparity after the end-Permian extinction, a clear example of an increase in disparity without a significant increase in taxonomic diversity. The results indicate that developmental constraints may not be responsible for the decreasing disparity in this group. The more likely scenario is that increasingly structured ecological guilds have made it much more difficult for large increases in disparity to occur.