Genetic variation has been reported for most of the major milk traits, and has been successfully exploited in genetic improvement programmes for sheep dairy production.
Sheep milk production represents a specialised commodity which has been developed across many breeding systems in either dual purpose, synthetic composite lines, or specialised dairy breeds such as the Awassi, Chios, Comisana, Lacaune, Laxta and Sarda Breeds. The multi-purpose nature of many sheep breeds and the highly specialised single purpose breeds, demonstrate the versatility and suitability of sheep production in a diverse set of production systems. Sheep represent an economically important agricultural resource in the global meat, fibre, and milk production systems of both the developed and developing world. In a cross-species comparison, we extended the meta-assembly by comparing QTL regions of sheep and cattle, which provided strong evidence for synteny conservation of QTL regions for milk, fat, protein and somatic cell score data between cattle and sheep. In addition, novel QTL were found on chromosomes 7, 8, 9, 14, 22 and 24. Those on chromosomes 3 and 20 are in strong agreement with the results reported here. Many of the QTL for milk production traits have been reported on chromosomes 1, 3, 6, 16 and 20. In preparation of a meta-analysis, all QTL results were compared with a meta-assembly of QTL for milk production traits in dairy ewes from various public domain sources and can be found on the ReproGen ovine gbrowser. A total of 15 significant ( P < 0.01) and additional 25 suggestive ( P < 0.05) QTL were detected across both single QTL methods and all traits. Derived milk traits for milk, fat, protein and lactose yield, as well as percentage composition and somatic cell score were used for single and two-QTL approaches using maximum likelihood estimation and regression analysis. From five previously proposed mathematical models describing lactation curves, the Wood model was considered the most appropriate due to its simplicity and its ability to determine ovine lactation curve characteristics. PLoS Biol, 6(3), e63.An (Awassi × Merino) × Merino backcross family of 172 ewes was used to map quantitative trait loci (QTL) for different milk production traits on a framework map of 200 loci across all autosomes. Loss of egg yolk genes in mammals and the origin of lactation and placentation. Why do mammals that do not lay eggs have non-functional egg yolk genes in their genomes? If these species do not lay eggs and didn’t evolve from ancestors that lay eggs, why would God have put these in their genomes? Why do some mammals share some identical loss-of-function mutations in their egg yolk genes genes?ġ. Together, these data suggest that the inactivation of egg yolk genes in placental and marsupial mammals is connected with the loss of their egg-laying ability through evolutionary time. Gallus gallus = chicken Monodelphis domestica = opossum Macropus eugenii and Wallabia bicolor = wallabies.īy contrast, at least one egg yolk gene is intact in the egg-laying platypus. Highlighted portions indicate loss-of-function mutations. DNA sequence alignment of egg-yolk genes. Similarly, the researchers found remnants of three egg-yolk genes ( VIT1, VIT2, VIT3) in the marsupials they studied (opossum, wallabies), with shared loss-of-function mutations in each (Figure 1), which, again, imply loss in a common ancestor.įigure 1. These mammals share some loss-of-function mutations in the genes, suggesting that the genes were inactivated in a common ancestor. So how might one test this hypothesis? David Brawand and his colleagues had the idea of looking at the genomes of mammals to see if they have any remnants of egg yolk genes.īesides monotremes, the remaining mammals can be divided into two general groups: (1) marsupials, whose offspring are born early in development and then finish developing in a pouch (marsupium), and (2) placental mammals, which develop with the help of a placenta connecting the mother to the fetus. When Brawand and his colleagues looked at the genomes of three placental mammals (human, dog, armadillo), they found remnants of two egg-yolk genes ( VIT1, VIT3), both of which possessed loss-of-function mutations.
#Platypus evolution meta plus
The fact that a few mammals lay eggs, plus that most other land-dwelling vertebrates do as well, points to the idea that the rest of mammals descended from egg-layers.
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