Dominant male Pygmy swordtails father most of brood

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Female Pygmy swordtails, Xiphophorus multilineatus, mate with several males at a single spawning but most of the offspring are fathered by a single fish, says new research.

Scientists from the University of Konstanz studied wild Pygmy swordtails and found that large dominant males display to females in a sigmoid pattern and that smaller, sneaker males sometimes crept in and fertilised the female with what they term "gonopodial thrusting", the fishy equivalent of a pelvic thrust.

By catching the females and analysing the DNA of their offspring with a molecular paternity test, the team were able to determine how many different mates the female had had, and how many of each fish had been fathered by particular males.

Interestingly, the results showed that the broods were fathered by between one and three males, but that the most dominant male fathered more than 70% of the fry.

The study shows that, in the case of X. multilineatus, the sneaker males aren't really getting very far in fertilising the females.

The authors say that their results also show that sperm competition might also have something to do with the paternity skew seen in the results.

For more details see the paper: Luo J, Sanetra M, Schartl M, Meyer A (2005) - Strong Reproductive Skew Among Males in the Multiply Mated Swordtail Xiphophorus multilineatus (Teleostei). J Hered. 2005 Mar 2.