This Week in PNAS
EVOLUTION
Bigger brains help birds in new environments
Larger brains are hypothesized to have evolved as an adaptation to cope with novel or changed environments, yet empirical support has been difficult to find. Daniel Sol et al. analyzed a global database of >600 cases of deliberate human efforts to introduce 195 bird species into new environments. The authors report that species with larger brains, relative to their body mass, tend to be more successful at establishing new populations. The authors controlled for variables that might affect a species' survival, such as native geographic range, fecundity, and broadness of diet. The researchers also searched the literature for reports of unusual behavior among newly introduced birds and found that brain size was positively correlated with establishment success through its effect on innovative behaviors, which suggests cognitive rather than noncognitive mechanisms behind the relationship between survival and brain size. Several explanations have been suggested, according to the authors, including the ability of a larger-brained animal to learn to reach less accessible foods or to deal with changes in social structure.
“Big brains, enhanced cognition, and response of birds to novel environments” by Daniel Sol, Richard P. Duncan, Tim M. Blackburn, Phillip Cassey, and Louis Lefebvre (see pages 5460–5465)
EVOLUTION
Vital codependence of aphids and symbiotic bacteria
The aphid and its bacterial tenant, Buchnera, have evolved to the point of extreme codependence: one cannot live or reproduce without the other. Atsushi Nakabachi et al. detail the genetic factors that underlie this intimate relationship. The obligate intracellular bacterium Buchnera provides the aphid host with essential amino acids and vitamins that are lacking in the aphid's nutrient-poor diet of phloem sap, and Buchnera appears to lack the ability to synthesize nonessential amino acids and phospholipids, suggesting that the host cell must compensate for these deficiencies. Nakabachi et al. isolated and assessed gene expression in bacteriocytes, the aphid host cells that harbor Buchnera. The authors found that genes involved in amino acid metabolism and transport, and encoding putative lysozymes, were highly expressed in aphid host cells. The expression of genes involved in transport and encoding lysozymes reveal previously unknown aspects of the symbiotic relationship. These results further suggest that bacteriocytes are actively involved in the exchange of amino acids between the aphid and bacterium.
Aphids' vital symbiosis with Buchnera.
“Transcriptome analysis of the aphid bacteriocyte, the symbiotic host cell that harbors an endocellular mutualistic bacterium, Buchnera” by Atsushi Nakabachi, Shuji Shigenobu, Naoko Sakazume, Toshiyuki Shiraki, Yoshihide Hayashizaki, Piero Carninci, Hajime Ishikawa, Toshiaki Kudo, and Takema Fukatsu (see pages 5477–5482)
GENETICS
Simplicity of histone code revealed
Histones, which package DNA in eukaryotes, play an important role in gene regulation. According to the histone code hypothesis, covalent posttranslational modification of histone tails, in this case, acetylation, influences gene regulation by altering transcriptional output. Michael Dion et al. tested this hypothesis by creating mutant yeast strains in which the four lysines (K) in the histone H4 tail of budding yeast, lysines 5, 8, 12, and 16, were replaced with arginine (R) in all 16 possible combinations. The replacement of lysine with arginine mimics the unacetylated lysine. The team analyzed the gene expression with DNA microarrays and showed that only the K16R substitution changed gene expression of a unique set of genes. Of the 125 genes whose expression differed, 67 showed a >2-fold change. The authors suggest that this result is consistent with a transcription mechanism involving a K16-binding protein. Mutations in the other three lysines did not uniquely affect gene expression. Instead, the total number of these K-to-R mutations caused incremental genome-wide shifts in expression. The authors conclude that the four H4 lysines produce a total of 8 transcription states rather than the possible 16, suggesting that the histone code is simpler than expected.
Gene expression of lysine mutants.
“Genomic characterization reveals a simple histone H4 acetylation code” by Michael F. Dion, Steven J. Altschuler, Lani F. Wu, and Oliver J. Rando (see pages 5501–5506)
GENETICS
Paternal allele-specific stimulatory G protein control region
Jie Liu et al. report that the 1A region of Gnas, the locus encoding the stimulatory G protein α-subunit (Gsα), contains a cis-acting, methylation-sensitive suppressor, which acts only on the paternal allele. Previous research has shown that Gsα is imprinted in a tissue-specific manner and is expressed primarily from the maternal allele in tissues such as renal proximal tubules. In other tissues, Gsα is expressed equally from both alleles. Liu et al. generated mice with a deletion of the 1A differentially methylated region (DMR), which lies upstream of Gsα-coding exons and is methylated on the maternal allele. The authors confirmed that the deletion on one allele did not affect 1A imprinting in the opposite allele and found that the deletion had no effect on the imprinting of promoters (Nesp and Nespas–Gnasxl) for alternative proteins expressed at the locus. In mice lacking the paternal but not maternal 1A DMR, Gsα mRNA and protein expression in renal proximal tubules increased by >50%. Neither maternal nor paternal 1A deletion affected Gsα expression in tissues where Gsα is normally expressed from both parental alleles. The paternal 1A deletion also reversed some aspects of the phenotype resulting from the loss of Gsα expression from the maternal allele.
Paternal 1A deletion reverses neonatal phenotype of maternal Gsα expression loss.
“Identification of the control region for tissue-specific imprinting of the stimulatory G protein α-subunit” by Jie Liu, Min Chen, Chuxia Deng, Déborah Bourc'his, Julie G. Nealon, Beth Erlichman, Timothy H. Bestor, and Lee S. Weinstein (see pages 5513–5518)
IMMUNOLOGY
InterMEM T cell state between naïve and effector
After passing through an intermediary state, naïve T cells concurrently differentiate into memory and effector cells, according to Susan Holmes et al. Previous research has shown that T cells require an encounter with antigen and other signals to undergo clonal expansion and differentiation, but the genetic events that underlie this process are not fully understood. Holmes et al. used FACSorting to isolate naïve, effector, and memory CD8+ T cells, based on the expression of CD27 and CD45RA, from the peripheral blood of 10 subjects. The researchers identified 156 genes differentially expressed among the three T cell subsets. By multivariate analysis, they were able to discern an intermediary state between the three cell types, termed InterMEM, which was closer to the memory T cell state than to naïve or effector states. This expression pattern, the researchers argue, supports the parallel differentiation model, in which naïve T cells differentiate into effector and memory cells simultaneously. In addition to CD27 and CD45RA, the authors report other biomarkers able to further discriminate among the three T cell subsets: syntaphilin and IFN-γ receptor 2 for naïve cells; vinculin and granzyme B for effector cells; and T cell receptor-interacting molecule and granzyme K for memory cells.
Hierarchical clustering of T cell arrays.
“Memory T cells have gene expression patterns intermediate between naïve and effector” by Susan Holmes, Michael He, Tong Xu, and Peter P. Lee (see pages 5519–5523)
