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. 2012 Jul 27;3:262. doi: 10.3389/fmicb.2012.00262

FIGURE 3.

FIGURE 3

Domestication from LTR retrotransposons and ERVs in mammals. The acquisition of SASPase occurred in a common mammalian ancestor. PEG10 was domesticated in a common therian ancestor while PEG11/RTL1, SIRH3–11, and PNMA1–19 were domesticated in a common eutherian ancestor with subsequent loss of some of the PNMA genes in rodents. The ESCAN domain was domesticated in lower vertebrates and its transition to the SCAN domain took place by combining with the zinc finger and/or KRAB motifs which had already occurred in certain reptiles. In any event, the expansion of SCAN family is obvious in the eutherians. SIRH12 and PNMA-MS1 are derived from marsupialspecific domestication events. The SYNCYTINs were independently recruited in several different mammalian lineages. The eutherians and marsupials are viviparous, having chorioallantoic and choriovitelline placentas (yolk sac placentas), respectively. Both PEG10 and PEG11/RTL1 are essential for the proper formation of efficient chorioallantoic placentas.