Alternative life histories shape brain gene expression profiles in males of the same population

Abstract

Atlantic salmon (Salmo salar) undergo spectacular marine migrations before homing to spawn in natal rivers. However, males that grow fastest early in life can adopt an alternative 'sneaker' tactic by maturing earlier at greatly reduced size without leaving freshwater. While the ultimate evolutionary causes have been well studied, virtually nothing is known about the molecular bases of this developmental plasticity. We investigate the nature and extent of coordinated molecular changes that accompany such a fundamental transformation by comparing the brain transcription profiles of wild mature sneaker males to age-matched immature males (future large anadromous males) and immature females. Of the ca. 3000 genes surveyed, 15% are differentially expressed in the brains of the two male types. These genes are involved in a wide range of processes, including growth, reproduction and neural plasticity. Interestingly, despite the potential for wide variation in gene expression profiles among individuals sampled in nature, consistent patterns of gene expression were found for individuals of the same reproductive tactic. Notably, gene expression patterns in immature males were different both from immature females and sneakers, indicating that delayed maturation and sea migration by immature males, the 'default' life cycle, may actually result from an active inhibition of development into a sneaker.

Figure 1

Physiological and life-history differences between early maturing sneakers and immature males of the same age. Juveniles that exhibit increased growth/size and condition during spring mature into sneakers during the summer/early autumn (‘maturation’ label), in time for spawning of large anadromous individuals. References: 1 (Rowe & Thorpe 1990), 2 (Whalen & Parrish 1999), 3 (Aubin-Horth & Dodson 2004), 4 (Amano et al. 1995), 5 (Antonopoulou et al. 1999), 6 (Mayer et al. 1991), 7 (Saunders et al. 1982), 8 (Mayer et al. 1998), 9 (Moore & Scott 1991), 10 (Waring et al. 1996), 11 (Arndt 2000).

Figure 2

Microarray hybridization design. RNA extracted from the brains of immature females (F), immature males (I) and mature sneaker males (S) Atlantic salmon individuals 1–4. Arrow-heads represent cy5-dye labelling and arrow tails cy3-dye labelling.

Figure 3

Proportion of genes from 17 biological processes up-regulated in (a) sneaker males and (b) immature males. A biological process was assigned to 135 of 235 genes upregulated in sneakers and to 145 of 197 genes upregulated in immature males, after removing spots without sequence information (sneaker males n=6, immature males n=5), no annotated hits (sneaker males n=38, immature males n=25), and unknown function (sneaker males n=56, immature males n=22). Note that some processes are unique to one tactic and that size of pie section representing a given biological process cannot be compared between sneaker and immature male figures, as this size is dependent on other categories found for this tactic.

Figure 4

Quantitative real-time PCR comparison of mean gene expression in the brains of immature females (F), immature males (I) and mature sneaker males (S). 1/cT (cycle threshold) value is directly proportional to initial amount of gene product. Vertical bars represent the 95% confidence interval and letters results of a posteriori tests, with same letter indicating no difference between groups at p=0.05. Both microarray and Q-RT-PCR showed prolactin and pro-opiomelanocortin (POMC A) to be upregulated in sneaker male brains (see Electronic Appendix, supplementary table S1). Additionally, Q-RT-PCR gave consistent results for genes such as somatostatin (see Supplementary table S1), which according to the microarray, were expressed at similar levels in the brains of all three phenotypes.

Figure 5

Cluster analysis of similarity of individual gene expression profiles of mature sneaker males 1–4, immature males 1–4 and immature females 1–4. Distance matrix based on Pearson correlation. Numbers at branch nodes in the consensus tree represent the percentage of trees that showed this clustering configuration of individuals when those trees were built using 1000 bootstrapped datasets (see §2).