Linkage arrangement in the vitellogenin gene family of Xenopus laevis as revealed by gene segregation analysis

Vitellogenin gene expression in marine mussels exposed to ethinylestradiol: No induction at the transcriptional level

Vitellogenin (Vtg), a large multidomain protein precursor of egg-yolk proteins, is used as an endocrine disruption biomarker in fish, and in the last decades, its use has been extended to invertebrates like mollusks. However, it remains unclear whether invertebrate endocrine system produces Vtg in response to estrogens, like it occurs in oviparous vertebrates. In a previous study, no evidence of induction of Vtg expression at protein level was found in gonads of the marine mussel Mytilus galloprovincialis after exposure to the estrogenic chemical 17α-ethinylestradiol (EE2). In the present follow-up study, it was investigated whether there is any effect of EE2 on Vtg abundance at transcriptional level in M. galloprovincialis gonads. To this aim, RT-qPCR analysis targeting three different domains of Vtg transcript was performed on gonads of mussels that were exposed either 4 or 24 days to 100 ng/L EE2. In addition, several reference genes were analysed and a selection of these for potential use in further RT-qPCR analyses on mussel male and female gonads is provided. Results showed higher expression in females than in males for the three analysed Vtg domains, and no evidence of Vtg mRNA induction due to EE2 either in females or males. The present results, together with those obtained from previous analysis at protein level, support that Vtg is not an adequate biomarker for xenoestrogenicity in marine mussels. Additionally, nucleotide sequences of Vtg transcripts of three closely-related species from Mytilus edulis complex (M. galloprovincialis, M. edulis and M. trossulus) are provided and compared with Vtg sequences from other mollusk species to assess the level of conservation and evolutionary relationships among species.

Loss of egg yolk genes in mammals and the origin of lactation and placentation

Embryonic development in nonmammalian vertebrates depends entirely on nutritional reserves that are predominantly derived from vitellogenin proteins and stored in egg yolk. Mammals have evolved new resources, such as lactation and placentation, to nourish their developing and early offspring. However, the evolutionary timing and molecular events associated with this major phenotypic transition are not known. By means of sensitive comparative genomics analyses and evolutionary simulations, we here show that the three ancestral vitellogenin-encoding genes were progressively lost during mammalian evolution (until around 30–70 million years ago, Mya) in all but the egg-laying monotremes, which have retained a functional vitellogenin gene. Our analyses also provide evidence that the major milk resource genes, caseins, which have similar functional properties as vitellogenins, appeared in the common mammalian ancestor ∼200–310 Mya. Together, our data are compatible with the hypothesis that the emergence of lactation in the common mammalian ancestor and the development of placentation in eutherian and marsupial mammals allowed for the gradual loss of yolk-dependent nourishment during mammalian evolution.

Egg yolk contains the nutrients required for the development of the nonmammalian vertebrate embryo. These nutrients derive by and large from a single set of proteins, vitellogenins, which are produced in the liver and provide or transport amino acids, lipids, phosphorous, and calcium to the egg. Mammals have evolved new nutritional resources for their developing and early offspring, such as lactation and placentation. However, the evolutionary timing and molecular events associated with this major phenotypic transition are not well understood. In this study, we have investigated the evolutionary fate of the three ancestral vitellogenin-encoding genes in mammals. Using detailed evolutionary analyses of genomes from the three major mammalian lineages (eutherian “placental” mammals, marsupials, and monotremes), we found that these genes progressively lost their functions and became pseudogenes relatively recently during mammalian evolution (the most recent inactivation event occurred roughly 30–70 million years ago). Monotremes, which lactate yet lay small parchment-shelled eggs, even retained a functional vitellogenin gene, consistent with their intermediate reproductive state. Our analyses also provide evidence that the major milk resource genes, caseins, which have similar functional properties as vitellogenins, appeared in the common mammalian ancestor ∼200–310 million years ago. Based on our data, we suggest that the emergence of the alternative resources for the mammalian young—lactation and then placentation—only gradually reduced the need for egg yolk resources (and hence functional vitellogenin genes) in mammals.

The major egg yolk genes, those that express vitellogenins, appear to have progressively lost their functionality during mammalian evolution, probably due to the emergence of the mammalian-specific developmental nourishment resources, lactation, and placentation.

Duplication of the MHC-linked Xenopus complement factor B gene

We have previously reported the molecular cloning of the mammalian major histocompatibility complex (MHC) class III gene, complement factor B (Bf) from Xenopus laevis, and linkage of the gene to the frog MHC. Here, we estimated the copy number of the Xenopus Bf gene by genomic Southern blotting analysis and demonstrated that Xenopus laevis has two copies of the Bf gene. Both genes co-segregated with the MHC-linked HSP70 genes among 19 offspring of an f/r x f/r cross, indicating a close linkage of the two Bf genes to the frog MHC. Both genes are transcribed and contain open reading frames. When compared with the previously determined cDNA sequence (Xenopus Bf A), the predicted amino acid sequence of the second cDNA species (Xenopus Bf B) shows 82% overall identity. Polymerase chain reaction analysis indicated that all of the partially inbred frogs with the f, r, g, and j MHC haplotypes, as well as 12 outbred frogs tested have both Bf genes, suggesting that the duplicated Bf genes are stable genetic traits in Xenopus laevis.

Hsp70 genes are linked to the Xenopus major histocompatibility complex

Some of the inducible forms of the heat shock protein 70 (Hsp70) gene family are encoded in the class III region of the major histocompatibility complex (MHC) of mammals. This study was undertaken to determine whether Hsp70 genes are linked to the MHC of Xenopus, an amphibian last sharing a common ancestor with mammals 300-350 million years ago. Segregation analyses involving seven haplotypes demonstrated the linkage of two or three inducible Hsp70 genes to the frog MHC. Another Hsp70 gene is not closely linked to the MHC. We conclude that the physical association of MHC class I and class II genes with Hsp70 genes is ancient.

Estradiol-induced vitellogenin gene expression in a teleost fish, Oreochromis aureus

Vitellogenesis presents a versatile model for the study of hormone-induced gene expression. We report here the effects of estradiol-17 beta-propionate on vitellogenin gene expression in male Oreochromis aureus, a teleost fish. Vitellogenin mRNA of 6500 nucleotides has been elucidated from the livers of female and estradiol-treated male O. aureus. By hybridization with a specific O. aureus cDNA probe, the vitellogenin mRNA transcript was detected as early as 1 hr following primary and secondary estradiol-stimulations, although for the latter, the rate of accumulation of vitellogenin-specific mRNA was 20-fold higher. The vitellogenin mRNA peaked at 72 and 48 hr, respectively, for primary and secondary stimulations. At the translational level, the increase in plasma vitellogenin was further enhanced during the secondary stimulation. There was a distinct shift in the peak of plasma vitellogenin from Day 14 in the primary induction to Day 3 in the secondary stimulation. The plasma vitellogenin presented in two forms, 300 and 500 kDa, both of which were immunologically confirmed by Western blot analysis to be vitellogenin proteins.

Regulation of oogenesis: the piscine receptor for vitellogenin

The receptor-mediated uptake of vitellogenin (VTG), a plasmatic lipophosphoglycoprotein, is crucial for oocyte growth in egg-laying animals. The plasma membrane receptor for VTG was characterized from oocytes of coho salmon, Oncorhynchus kisutch. In direct binding studies, the receptor exhibited high affinity (Kd, 180 nM) for salmonid VTG, and by ligand blotting with radiolabelled VTG it was visualized as a protein with an apparent Mr of 100,000, under non-reducing conditions. The fish VTG receptor was shown to share key structural elements with VTG receptors from chicken and Xenopus laevis. Namely, cross-reactivity at the level of ligand recognition was observed among VTG receptors from these species and immunological relatedness was demonstrated by immunoblotting with anti-chicken VTG receptor antibodies. In addition, as in chicken and Xenopus, binding of VTG to fish oocyte receptors was shown to be mediated by the lipovitellin domain of VTG. These results clearly indicate that regulation of oocyte growth at the level of yolk formation has been accomplished by the conservation of structural features of receptors required for internalization of VTG.

Genetic mapping in Xenopus laevis: eight linkage groups established

Inheritance of alleles at 29 electrophoretically detected protein loci and one pigment locus (albinism) was analyzed in Xenopus laevis by backcrossing multiply heterozygous individuals generated by intersubspecies hybridization. Pairwise linkage tests revealed eight classical linkage groups. These groups have been provisionally numbered from 1 to 8 in an arbitrarily chosen order. Linkage group 1 includes ALB-2 (albumin), ADH-1 (alcohol dehydrogenase), NP (nucleoside phosphorylase), and ap (periodic albinism). Linkage group 2 contains ALB-1 and ADH-2, and probably is homeologous to group 1. Linkage group 3 comprises PEP-B (peptidase B), MPI-1 (mannosephosphate isomerase), SORD (sorbitol dehydrogenase), and mIDH-2 (mitochondrial isocitrate dehydrogenase). Linkage group 4 contains GPI-1 (glucosephosphate isomerase) and EST-4 (esterase 4). Linkage group 5 contains GPI-2 and PEP-D (peptidase D). Linkage group 6 comprises ACP-3 (acid phosphatase), sME (cytosolic malic enzyme), and GLO-2 (glyoxalase). Linkage group 7 consists of sSOD-1 (cytosolic superoxide dismutase), GPD-2 (glycerol-3-phosphate dehydrogenase), mME (mitochondrial malic enzyme), and the sex determining locus. Linkage group 8 includes FH (fumarate hydratase) and TRF (transferrin). Recombination frequencies between linked loci showed differences related to the genomic constitution (parental subspecies) and to the sex of the heterozygous parent. Independent assortment was observed between the duplicate ALB loci. This is true for the duplicate ADH, GLO, and MPI loci as well, supporting the view that these genes have been duplicated as part of a genome duplication that occurred in the evolutionary history of X. laevis. Comparative analysis of genetic maps reveals a possible conservation of several linkages from the Xenopus genome to the human genome.

Rudimentary phosvitin domain in a minor chicken vitellogenin gene

We have determined the nucleotide sequence and the derived amino acid sequence of the phosphoprotein-encoding region of the chicken vitellogenin III gene. The sequence of this minor vitellogenin could be aligned with exon 22 up to exon 27 of the previously sequenced major vitellogenin II gene (van het Schip et al., 1987). The exon 23 and 25 sequences are rich in serine codons (26% and 41%, respectively), and this region encodes at least one of the small egg yolk phosphoproteins. The major egg yolk phosphoprotein, phosvitin, is encoded by the analogous region in vitellogenin II. Comparison of the vitellogenin II and vitellogenin III sequences shows a great reduction in the size of the putative exon 23 of the latter (321 base pairs as opposed to 690). The number of serine codons is also drastically reduced from 124 in exon 23 of the vitellogenin II gene to 28 in vitellogenin III. The grouping of synonymous serine codons, as has hitherto been observed in sequenced vitellogenin phosphoproteins, has been maintained in vitellogenin III. A putative asparagine-linked N-glycosylation site which was conserved in the chicken vitellogenin II and the Xenopus laevis vitellogenin A2 gene, at the beginning of exon 23, is also present in vitellogenin III. The two chicken vitellogenins show a low conservation in the phosphoprotein-encoding region (average 33%, at the protein level) compared to that in the peripheral sequences (58% identity), which indicates that it is a rapidly evolving domain of the vertebrate vitellogenin gene.

cis- and trans-acting elements of the estrogen-regulated vitellogenin gene B1 of Xenopus laevis

Vitellogenin genes are expressed under strict estrogen control in the liver of female oviparous vertebrates. Gene transfer experiments using estrogen-responsive cells have shown that the 13 bp perfect palindromic element GGTCACTGTGACC found upstream of the Xenopus laevis vitellogenin gene A2 promoter mediates hormonal stimulation and thus, was called the estrogen-responsive element (ERE). In the Xenopus vitellogenin genes B1 and B2 there are two closely adjacent EREs with one or more base substitutions when compared to the consensus ERE GGTCANNNTGACC. On their own, these degenerated elements have only a low or no regulatory capacity at all but act together synergistically to form an estrogen-responsive unit (ERU) with the same strength as the perfect palindromic 13 bp element. Analysis of estrogen receptor binding to the gene B1 ERU revealed a cooperative interaction of receptor dimers to the two adjacent imperfect EREs which most likely explains the synergistic stimulation observed in vivo. Furthermore, a promoter activator element located between positions --113 and --42 of the gene B1 and functional in the human MCF-7 and the Xenopus B3.2 cells has been identified and shown to be involved in the high level of induced transcription activity when the ERE is placed at a distance from the promoter. Finally, a hormone-controlled in vitro transcription system derived from Xenopus liver nuclear extracts was exploited to characterize two additional novel cis-acting elements within the vitellogenin gene B1 promoter. One of them, a negative regulatory element (NRE), is responsible for repression of promoter activity in the absence of hormone. The second is related to the NF-I binding site and is required, together with the ERE, to mediate hormonal induction. Moreover, we detected three trans-acting activities in Xenopus liver nuclear extracts that interact with these regions and demonstrated that they participate in the regulation of the expression of the vitellogenin promoter in vitro.