Egg yolk gene loss was mammals' gain

Written byElie Dolgin

| 2 min read

Mammals lost their egg yolk genes after acquiring genes for milk proteins, according to a linkurl:study;https://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0060063/ published yesterday in PLoS Biology. The results pinpoint an important step in how mammals evolved, the authors say. Lactation is "what makes us mammals, basically," said linkurl:Henrik Kaessmann,;https://www.unil.ch/cig/page7858_en.html/ who led the study. "Using egg yolk genes as markers, we found a unique way to put a timeframe on how key transitions in mammals occurred." There are three types of mammals: true placental mammals, linkurl:marsupials;https://www.the-scientist.com/news/home/53187/ and monotremes. Though each type nourishes its young in a different way, they all use milk to some extent, and their eggs have far less yolk than their reptilian and bird-like ancestors. But in the evolution of mammals, there's a longstanding "chicken and egg" question, or rather, a milk and egg question: What came first in the mammalian lineage — genes involved in lactation, or the loss of genes for making egg proteins? Now researchers at the University of Lausanne in Switzerland have cracked the problem. Kaessmann and his colleagues compared the sequences of genes encoding linkurl:vitellogenin,;https://www.the-scientist.com/news/home/52926/ an essential egg yolk protein, in the different mammalian lineages. They found that the three genes present in the mammalian ancestor were progressively lost in all lineages except for the monotremes, which have retained one working gene. These primitive mammals, which include platypuses and echidnas, lactate, yet lay small, parchment-shelled eggs. So the presence of both functional and non-functional vitellogenin genes is consistent with this intermediate reproductive state, said Kaessmann. The researchers also found that all three groups of mammals shared major milk resource genes, called caseins, indicating that these genes arose over 200 million years ago, before the split of the mammalian lineages. Putting findings from both sets of genes together, Kaessmann argues that lactation in the common mammalian ancestor, followed by the emergence of linkurl:placentas;https://www.the-scientist.com/article/display/22857/ in some mammals, probably allowed for the loss of yolk-nourishment in mammals. "Everything makes sense," said linkurl:Jay Storz;https://bs-biosci.unl.edu/faculty/Storz/index.html/ of the University of Nebraska in Lincoln, who was not involved in the research. He told The Scientist he was "surprised that the progressive loss of the yolk genes coincided so nicely with the origins of lactation and placental development." linkurl:Nigel Finn;https://nigelfinn.com/ of the University of Bergen in Norway agreed that the evidence was "quite convincing," but he thinks the picture of mammalian evolution is still incomplete. The transition to mammalian reproduction was not just about nutrition, he argues, mammals had to overcome a water problem as well, and the study does not address the evolution of amniotic fluid, which nourishes the embryo and keeps it moist. "Yolk is a luxury, but water is essential," he said.

Egg yolk gene loss was mammals' gain

The Scientist ARCHIVES

Become a Member of

Meet the Author

Elie Dolgin

Related Research Resources

Epic Fail: Sea-Monkeys Sabotage Fieldwork

Research Resources

Podcasts

Webinars

Videos

Infographics

eBooks

Overcoming Immunotherapy Resistance in Liver Cancer

Optimizing NGS Library Preparation for Reliable Sequencing Data

Using TCR Repertoire Sequencing to Advance Immunology Research

Mapping Clonal Mosaicism in Aging Tissues

Products

Product News

Sino Biological Launches SuperNuclease ® Pro with Free Trial Program

Sino Biological Launches Precisely Characterized Full-Length p-Tau217 Protein to Advance Next-Generation Alzheimer’s Biomarker Assay Development

Resipher: Continuous Live-Cell Mitochondrial Respiration Monitoring in 96-Well Plates

The VAULT100 PRO: Inside the most advanced Stirling Ultracold ULT freezer ever built.