Trout surrogates could support single spawning salmon

Imagine you're beside a rocky stream in the mountains and you see a clutch of fish eggs. You think they belong to salmon or trout, but you're not a fish expert and how different could they be, anyway? Turns out, pretty different! If they are chinook salmon eggs, they're probably orphans. Chinook salmon (Oncorhynchus tshawytscha) spawn once in freshwater streams after a substantial migration back from the ocean and then die. If they are rainbow trout eggs, their parents are probably around. Rainbow trout (Oncorhynchus mykiss) spawn in freshwater streams multiple times during their life. Why some salmonids are single-spawners and others are multiple-spawners is not well understood. However, Goro Yoshizaki and colleagues at the Tokyo University of Marine Science and Technology and the University of Tsukuba, Japan, have uncovered a potential reason for this difference and it involves reproductive stem cells: the small but important cells that give rise to eggs and sperm.First, the scientists wondered what happens to these stem cells before and after spawning. When they looked at the immature gonads of rainbow trout (multiple-spawners) and chinook salmon (single-spawners) under the microscope, they saw lots of these stem cells – ready to turn into sperm or eggs. After the fish had spawned once, they checked again and saw stem cells in the ovaries and testes of rainbow trout, but not in those of chinook salmon. This suggested that single-spawning species lose their ability to produce eggs and sperm after one reproductive cycle, while this is retained in multiple-spawners.Next, they wondered whether the loss of these stem cells is a property of the cells themselves, or a result of signals coming from the fish. To tackle this question, the scientists collected the stem cells from chinook salmon and, using a tiny glass needle, transplanted them into rainbow trout. Specifically, larval rainbow trout genetically modified to have no reproductive stem cells of their own. In doing so, the stem cells of the single-spawner were now receiving signals from the body of a multiple-spawner. They then raised these rainbow trout with salmon stem cells to maturity and through several spawning seasons.When the recipient rainbow trout males started to produce sperm, they collected it and analysed the DNA. As they hoped, the trout were producing chinook salmon sperm, confirming that the transplant worked. Similarly, when the female trout started producing eggs, they were chinook salmon eggs. When these recipient rainbow trout mated, they produced fully chinook salmon offspring – as verified by their DNA, development and appearance. After this first spawning, the question remained: would the chinook salmon stem cells disappear after one spawning like they do in their natural host, or would their new rainbow trout host make them stick around for multiple spawning seasons?It turns out these rainbow trout could produce chinook salmon sperm or eggs for multiple spawning seasons! By examining the ovaries and testes under the microscope after spawning, the researchers confirmed that transplanted chinook salmon stem cells stuck around in recipient trout. After this years-long experiment, the scientists concluded that the fate of these reproductive germ cells after the first spawn is not a built-in property, but depends on signals from within the fish. Which signals? That is for future scientists to figure out.Now, imagine you're back at your stream, eyeing these eggs suspiciously. Are they the product of the single monumental spawning of chinook salmon or one of many clutches produced by a rainbow trout? Perhaps they are the chinook salmon offspring of two genetically modified, stem cell recipient rainbow trout created in an attempt to boost an endangered population of chinook salmon.