Females have a larger hippocampus than males in the brood-parasitic brown-headed cowbird

Females of the brood-parasitic brown-headed cowbird (Molothrus ater) search for host nests in which to lay their eggs. Females normally return to lay a single egg from one to several days after first locating a potential host nest and lay up to 40 eggs in a breeding season. Male brown-headed cowbirds do not assist females in locating nests. We predicted that the spatial abilities required to locate and return accurately to host nests may have produced a sex difference in the size of the hippocampal complex in cowbirds, in favor of females. The size of the hippocampal complex, relative to size of the telencephalon, was found to be greater in female than in mate cowbirds. No sex difference was found in two closely related nonparasitic icterines, the red-winged blackbird (Agelaius phoeniceus) and the common grackle (Quiscalus quiscula). Other differences among these species in parental care, migration, foraging, and diet are unlikely to have produced the sex difference attributed to search for host nests by female cowbirds. This is one of few indications, in any species, of greater specialization for spatial ability in females and confirms that use of space, rather than sex, breeding system, or foraging behavior per se, can influence the relative size of the hippocampus

Cross-laboratory quantification of BSA-BDP cross-linked peptide pairs.

<p><b>A.</b> Average cross-linked peptide response curve comparing data collected in the Bruce Lab and CSHL for 30 cross-linked peptide pairs. <b>B.</b> Scatter plot illustrating the R² values for linear regression analysis of the data shown in A and B.</p

Experimental outline.

<p><b>A.</b> Biological samples are prepared for qXL-MS comparing two or more conditions. The samples are treated with chemical cross-linker either as (1) a mixed sample if SILAC labeling was used or (2) as separate samples if carrying out a label free experiment or using isotopically labeled cross-linkers. Following the cross-linking reaction proteins are extracted, enzymatically digested, and subjected to various strategies (i.e. strong cation exchange and affinity chromatography) for enrichment cross-linked peptide pairs. <b>B.</b> LC-MS analysis of samples enriched for cross-linked peptide pairs is carried out. This consists of reversed phase chromatographic separation by LC followed by analysis by MS. The mass spectrometer is operated in PRM mode where an inclusion list of <i>m/z</i> values for the precursor ions of interest is used to target specific cross-linked peptides. The PRM mass spectrometric analysis used here consists of three steps including isolation of precursor ions, fragmentation by collision with neutral gasses, and detection of mass to charge ratios of the resulting fragment ions. <b>C)</b> Resulting MS2 data are converted into transition lists and imported into Skyline for analysis.</p

Quantification of BSA cross-linked peptide pairs with Skyline.

<p><b>A.</b> MS2 spectrum for the cross-linked peptide pair linking residues K235-K28 (ALK²³⁵AWSVAR_DTHK²⁸SEIAHR), obtained from a 500 ng injection of cross-linked BSA digest. <b>B.</b> Extracted ion chromatograms for the PRM transitions observed for the cross-linked peptide pair in A. <b>C.</b> Skyline generated bar plot illustrating the normalized peak areas for the cross-linked peptide pair linking K28-K235. Peak areas are shown for triplicate analyses of varying injection amounts (100, 200, 500, and 1000 ng cross-linked BSA digest). Bars are color coded to indicate the contribution of each individual transition to the total peak area and match the color scheme in panel B. </p