Xenotoca eiseni are live-bearing fish native to Mexico. X. eiseni are highly matrotrophic and the embryos develop within the ovary of the mother. During gestation, embryos increase in dry weight by up to 387 times the initial mature ova weight (Wourms and Hollenberg, 1995). In the ovary, embryos receive nutrients and expel waste through trophotaenia, extensions of the intestine (Fig. 1).
When studying the placenta of viviparous organisms, the mammalian placenta is often used as a model from which to base understanding. Albumin and a-fetoprotein (AFP) are serum binding proteins that assist in the transport of long-chain fatty acids across the placenta (Savu et al., 1981, Calvo et al., 1987). In mammals, albumin is a plasma protein produced by the maternal liver. AFP and albumin are both produced by the fetal liver; AFP is also synthesized by the fetal yolk sac. AFP is specific to the fetal extra embryonic membranes and is largely distributed among vertebrate species (Calvo et al 1987). These proteins have not yet been identified for X. eiseni. Detection of these two proteins would assist in understanding the placental nature of extaembryonic structures in live-bearing fish.
To study organ distribution, tissue distribution, and ontogenetic expression of the mRNA for AFP and albumin, mRNA will be isolated and converted to cDNA for gene cloning and sequencing. Using the published sequences of albumin and AFP from other fish, primers can be created for conserved regions of the genes.
The tissues of maternal ovary and liver at different stages of pregnancy will be homogenized for mRNA extraction. The tissues of whole embryos before and after organ development will be studied as well as the trophotaenia of the fish and the liver of late-stage fetuses for the expression of AFP and albumin.
I expect to find albumin mRNA in the maternal liver at early stages of pregnancy, before a fetal liver develops for albumin production.. If it is found that albumin is also expressed in the embryo at this stage it supports that it albumin is produced by the mother and transported into the ovarian lumen to be taken in by the embryos.. If albumin is then found within the trophotaenial tissue, it is strong evidence that trophotaenia play a major role in albumin transport to the underdeveloped embryos.
If AFP is present, I expect to find it in the tissue of underdeveloped embryos. Finding AFP at this phase will serve as indication that it is likely produced by the embryonic yolk sac. Detecting AFP will support our hypothesis that trophotaenia are playing a placental role in X. eisini fetal development. AFP has been found to be only expressed in mammalian and chicken placenta. It is specific to the fetal placenta and is produced by the yolk sac and then the fetal liver when it is developed. We will test to see if AFP is present in fetal X. eiseni tissues at different stage, and if so in which tissues.
Understanding of what the embryonic X. eiseni receives from the maternal system before organ development, and what is manufactured by the embryonic after organ development will give insight into the placental function of trophotaenia and the nature of fetal-maternal interactions in live bearing fishes.