Cloning of somatolactin alpha and beta cDNAs in zebrafish and phylogenetic analysis of two distinct somatolactin subtypes in fish

Abstract

Somatolactin (SL) is a pituitary hormone belonging to the growth hormone/prolactin superfamily, with recognizable homologues in all fish taxa examined to date. Although sequences from most fish share reasonably high sequence identity, several more highly divergent SLs have been reported. Goldfish SL and a second SL protein found in rainbow trout (rtSLP) are remarkably different from each other and also dissimilar to other SLs. It has been unclear whether rtSLP is a recent paralogue restricted to rainbow trout, or reflects a more ancient duplication of the SL gene, and whether it is related to the goldfish sequence. Here we report the cloning of two different zebrafish SL cDNAs, which share only 57·5% nucleotide and 47·7% deduced amino acid identities. One copy, designated zebrafish SL (zfSL ), displays a typical range of sequence similarity to most other SLs. The other copy, zebrafish SL (zfSL ), shows low identity to most other SLs; surprisingly, it is most similar to the divergent SL sequence from goldfish. The mRNAs of zfSL and zfSL were expressed specifically in two distinct regions of the pars intermedia in zebrafish. Cells expressing zfSL are located at the posterior pars intermedia, bordering the neurohypophysis, whereas zfSL is expressed in the anterior part of the pars intermedia, bordering the pars distalis. Phylogenetic analyses indicate that zfSL , goldfish SL and rtSLP all belong to the SL hormone family; however, along with the genes from eel and catfish, these divergent sequences form a group that is clearly distinct from all other SLs. These results suggest the presence of two distinct SL families, SL and SL , which may trace back to a teleost genome duplication prior to divergence of the cyprinids and salmonids.

Evolution of Receptors for Growth Hormone and Somatolactin in Fish and Land Vertebrates: Lessons from the Lungfish and Sturgeon Orthologues

“…Therefore, unlike SLR, SL should have existed prior to the FSGD, which produced SLa and SLb paralogs in teleosts. Whereas the presence of SLb hass been reported for some teleost species other than zebrafish (Zhu et al 2004), we could not find SLb in the genomes of medaka and fugu.…”

“…While the duplicated GH seems to have been lost from the genomes of medaka, fugu, or zebrafish, the duplicated SL is retained in zebrafish (Fig. 3) and some other teleost lineages (Zhu et al 2004). The duplicated SLa and SLb have limited sequence similarities (\50%) and the receptor for SLb has not been investigated in any species to date.…”

Parathyroid hormone-related protein in teleost fish

“…Interestingly, in the sea bream pituitary gland, PTHrP is found only in the SL-producing cells that would be similar to the recently described SLa cells in zebrafish (Zhu et al, 2004). Indeed, in an earlier study, Ingleton et al (1998) reported that in sea bream, PTHrP and SL are both located in PAS positive cells and that some cells do contain both PTHrP and SL.…”

How to make a teleost adenohypophysis: Molecular pathways of pituitary development in zebrafish

“…The PI consists of corticotropes and melanotropes, which generate -melanocyte stimulating hormone (-MSH) (Liu et al, 2003). As a specific endocrine feature, teleosts share an additional adenohypophyseal cell type: the Somatolactin producing cells, which can be found in the PD of the embryonic and in the PD as well as PI of the adult AH (Zhu et al, 2004, Lopez et al, 2006.…”