Here are a few resources that might help out.

Scribner et al. 2001. Hybridization in freshwater fishes: a review of case studies and cytonuclear methods of biological inference. Reviews in Fish Biology and Fisheries.

Hubbs. 1955. Hybridization between fish species in nature. Syst. Zool.

Dowling and Secor. 1997. The role of hybridization and introgression in the diversification of animals. Ann. Rev. Ecol. Sys.

Thank you very much, Justin. I am also interested in your articles.

Best regards,

Yen

The more similar they are the easier it is to produce hybrids. A ggod example is with American catfish. Channel and blue have the same number and hybridize fairly easily. White has fewer chromosomes and the size is different. White X channel or blue hybrids are harder to make and the offspring are often deformed. Even if a dissimilar karyotype does not stop the productionof hybrids, the offpsring can be "mules" in that they are sterile. A good example is golden shiner X rudd hybrids.

Dear Dr. Douglas, your answer also explain the cases of Clarias hybrids, where the karyotype of Clarias gariepinus (Cg) is 2n = 56 (Teugels et al., 1992), and that of C. macrocephalus (Cm) is 2n = 52 (Sittikraiwong, 1987), and Cm x Cg hybrids have 2n = 54; (Visoottiviseth et al., 1997). Similarly, hybrids between Clarias gariepinus and Heterobranchus longifilis (Hl) (2n = 52) also have 2n = 54 chromosomes (Teugels et al., 1992)

My next questions are, how do chromosomes of hybrids matter? Hybrids with odd numbers of chromosome are sterile, while those with even number can be fertile? Is this always the case for many fish species?

Do you use channel x blue catfish hybrids for commercial farming in the US? Do you check if introgression has occurred in wild populations of each species?

Thank you,

To get viable hyrbids you have to have chromome compatability of some sort. Even if you have parents with the same numbers, if the morphology of the chromosomes are different, they cannot pair properly. This might cause fertilization problems. If it does not, it might cause the hybrids to be sterile. The US catfish industry is moving to channel X blue hybrids and there are many papers about this. A good place to start is with the work being done at Auburn. Amazingly, backcrossing the hybrids to the parent species does not produce good offspring.

Thank you very much for your explanation.