Salmon give birth to trout, suspect paternity

Why must scientists play with salmons' heads like this:

Researchers have succeeded in making salmon couples give birth to trout -- using a technique that they argue could help to preserve rare species of fish.

Goro Yoshizaki and his colleagues at the Tokyo University of Marine Science and Technology in Japan had previously shown that male salmon could be injected with cells from closely-related trout to produce viable trout sperm. When the sperm were introduced to trout eggs, healthy trout offspring were produced...

Now the researchers have taken the work a step further, showing that salmon can be not only the biological fathers but also the mothers of trout offspring. The new work, published in Science, shows how two sterile masu salmon (Oncorhynchus masou) can together produce nothing but healthy rainbow trout (O. mykiss).

The technique relies on the injection of trout spermatagonia -- the early, stem-cell stage of sperm -- into salmon embryos, so that the growing salmon produce trout sperm and eggs. The technique could be very useful for storing back-up genetic material of different fish species that are today under threat, because spermatagonia can be easily cryopreserved, says David Penman, a fish geneticist at the University of Stirling, UK.

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Male salmon: "Honey, I have noticed that all of our children appear to be trout?"
Female salmon: "Yes, dear. I had noticed that as well."
Male salmon: "Do you have anything you want to tell me about that..."
Female salmon: "Um...no...can't think of anything..."
Male salmon: "Maybe you want to comment about how the mailman is a trout?"

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Umm... Jake... how can the male and female salmon know what their offspring are? Aren't salmon (even the O. masou species greatly semelparous (i.e., they die after reproducing)? In any case, aren't these two species both part of the same Oncorhynchus genus? (Not to berate their work, of course.)