Evolution of Pinnipedia lactation strategies: a potential role for alpha-lactalbumin?

Abstract

Despite the considerable variation in milk composition found among mammals, a constituent common across all groups is lactose, the main sugar and osmole in most eutherians milk. Exceptions to this are the families Otariidae (fur seals and sea lions) and Odobenidae (walruses), where lactose has not been detected. We investigated the molecular basis for this by cloning alpha-lactalbumin, the modifier protein of the lactose synthase complex. A mutation was observed which, in addition to preventing lactose production, may enable otariids to maintain lactation despite the extremely long inter-suckling intervals during the mother's time at sea foraging (more than 23 days in some species).

Figure 1

(a) Genomic sequence alignments of intron 3/exon 4 boundary of α-LA from subantarctic (SFS), Cape (CFS) and northern fur seal (NFS), California (CSL) and Australian sea lion (ASL), Weddell (WS) and Ringed seal (RS), domestic dog (NP_001003129) and walrus (WAL). The mutation is bold and shaded in otariids α-LA, and dashes indicate the deletion in walrus α-LA. Exon 4 is underlined and bold. (b) Gene structure of α-LA (ALA) from domestic dog, Weddell seal and the two Cape fur seal variants (CFS (l), long; CFS (s), short). Intron 3 splice sites are shown. (c) Protein alignment of α-LA from Cape fur seal (long and short), Weddell seal, domestic dog (Q9N2G9), cow (P00711) and walrus (SFS and CSL α-LA proteins found to be identical to CFS; electronic supplementary material). Residues essential for lactose synthesis are underlined.

Figure 2

Generalized diagram of the different lactation strategies undertaken by the three families of pinnipeds. The otariid and phocid reproductive cycle is generally annual, whereas odobenid lactation can last up to two years with the pup accompanying its mother out to sea.