Evolution of the Yellow/Major Royal Jelly Protein family and the emergence of social behavior in honey bees

Abstract

The genomic architecture underlying the evolution of insect social behavior is largely a mystery. Eusociality, defined by overlapping generations, parental brood care, and reproductive division of labor, has most commonly evolved in the Hymenopteran insects, including the honey bee Apis mellifera. In this species, the Major Royal Jelly Protein (MRJP) family is required for all major aspects of eusocial behavior. Here, using data obtained from the A. mellifera genome sequencing project, we demonstrate that the MRJP family is encoded by nine genes arranged in an approximately 60-kb tandem array. Furthermore, the MRJP protein family appears to have evolved from a single progenitor gene that encodes a member of the ancient Yellow protein family. Five genes encoding Yellow-family proteins flank the genomic region containing the genes encoding MRJPs. We describe the molecular evolution of these protein families. We then characterize developmental-stage-specific, sex-specific, and caste-specific expression patterns of the mrjp and yellow genes in the honey bee. We review empirical evidence concerning the functions of Yellow proteins in fruit flies and social ants, in order to shed light on the roles of both Yellow and MRJP proteins in A. mellifera. In total, the available evidence suggests that Yellows and MRJPs are multifunctional proteins with diverse, context-dependent physiological and developmental roles. However, many members of the Yellow/MRJP family act as facilitators of reproductive maturation. Finally, it appears that MRJP protein subfamily evolution from the Yellow protein family may have coincided with the evolution of honey bee eusociality.

Figure 1.

Intron/exon structure of honey bee mrjp and yellow genes. Exons are depicted as black boxes, to scale. To emphasize structural similarities among mrjp genes, introns are depicted with empty boxes of uniform size, displaying actual sizes in base pairs. The intron/exon structure of the yellow-e3 gene is very similar to that of the mrjp genes.

Figure 2.

Phylogeny of the Yellow/MRJP protein family. An unrooted tree was constructed with aligned protein sequences from bees, flies, and bacteria using neighbor-joining (see Methods). The Yellow/MRJP protein family is composed of numerous color-coded subfamilies, each characterized by a group of closely homologous proteins. One of these subfamilies, the MRJPs, has thus far only been found in honey bees. Other subfamilies characterized by similar protein sequences are also known to have similar functions, and are so labeled. (AmY) Apis mellifera yellow; (DmY) Drosophila melanogaster yellow; (Phlepa) Phlebotomus papatasi; (Lutzlo) Lutzomyia longipalpis; (S.ba) Shewanella baltica; (S.aver) Streptomyces avermitilis; (G.met) Geobacter metallireducens; (G.xy) Gluconobacter oxydans; (N.crassa) Neurospora crassa; and (D.rad) Deinococcus radiodurans.

Figure 3.

Genomic landscape of genes encoding MRJPs. Four scaffolds from the A. mellifera genome project were joined by PCR amplification and sequencing, forming an 883-kb interscaffold (see Results). All 10 mrjp genes are aligned in a tandem array ∼60 kb in size. This, in combination with sequence similarity, suggests a recent series of duplication events in the formation of the MRJP protein subfamily. The mrjp array is flanked by five yellow genes, all but one of which read in the same direction as the mrjp loci. The orientation of the yellow-g and yellow-g2 genes suggests the presence of a shared, bidirectional promoter.

Figure 4.

Developmental-stage-, sex-, and caste-specific expression of a subset of yellow and mrjp genes. Shown are data from yellow-f, yellow-g, and yellow-h, and also mrjp-1, mrjp-3, and mrjp5. RT-PCR products were run on gels for visualization of gene expression. The primers are listed in Supplemental Table S3. The expression of mrjp5 was examined by Northern blotting using a PCR-amplified insert of a cDNA clone as a probe (nucleotides 15–1942 of NM_001011599). See Methods for more details.