THE GONADS AND REPRODUCTION OF THREE INTERGENERIC SUNFISH (FAMILY CENTRARCHIDAE) HYBRIDS.

The sterility of species hybrids has been known since the time of the ancient Greeks. Aristotle discussed at length the sterility of the mule (Dobzhansky, 1951). Hybrid sterility due to abnormal gonad development, especially in the male, is very common in viable F1 species hybrids in animals (Stebbins, 1966). The literature on hybrid sterility is extensive, and comprehensive reviews by Dobzhansky (1951), White (1954), and Stebbins (1958) are available. In the class Osteichthyes, the study of hybrid sterility is difficult because the chromosomes are, in general, too small and too numerous to permit detailed study (White, 1954). Therefore most of the studies have been concerned with gross and histological examination of the gonads and attempts at obtaining F2 and backcross offspring. In the Centrarchidae (sunfish family) there have been conflicting reports concerning the sterility of species hybrids. Earlier workers (Hubbs and Hubbs, 1932; 1933) concluded that with rare exceptions all species hybrids within the genus Lepomis were sterile. Ricker (1948) was apparently the first author to find that all centrarchid hybrids were not sterile. Since that time an increasing number of authors have found fertile centrarchid hybrids (Lagler and Steinmetz, 1957; Childers and Bennett, 1961; Smitherman and Hester, 1962; Childers, 1967). Roberts (1964) found karyotypic polymorphism in Lepomis cyanellus. He suggested that the conflicting reports might be attributed to the use of fish of the same species but with different karyotypes. Becak et al. (1966) have demonstrated intraindividual chromosomal polymorphism in Lepomis cyanellus. Childers (1967) found the following intergeneric centrarchid hybrids to be fertile: Chaenobryttus gulosus X Lepomis cyanellus, L. cyanellus X C. gulosus, L. microlophus X C. gulosus and C. gulosus X L. macrochirus. Birdsong and Yerger (1967) found that in the latter hybrid some of the testes were represented by mere strands of tissue, but three of 24 male hybrids produced normal spermatozoa. One hybrid female had well developed ovaries with a large number of apparently normal eggs. White (1954) suggested that experimental hybridization may reveal the extent and nature of the isolating mechanisms separating two species. It was the purpose of the present investigations to study the reproduction of the following intergeneric hybrids: Micropterus salmoides (the largemouth bass) X Chaenobryttus gulosus (the warmouth), C. gulosus X M. salmoides, and C. gulosus X Lepomis macrochirus (the bluegill), with special reference to hybrid sterility and the interpretation of this information with respect to the isolating mechanisms separating these species. In the above hybrid designations and throughout this paper the female is listed first. According to present centrarchid taxonomy the first two hybrids are intertribal crosses, and the last hybrid is an intratribal cross. 1 Based on a portion of a thesis submitted to the Department of Zoology, North Carolina State University at Raleigh, in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 2 Present address: Department of Biology, Western Carolina University, Cullowhee, N. C. 28723.