FERTILITY AND VIABILITY IN INTERGENERIC PHEASANT HYBRIDS

In spite of the fact that fertility and viability are to a high degree independently determined, they will be considered together in the present report. This procedure is believed to be justified because, as has been abundantly shown by a wealth of empirical data from many groups of animals and plants, both phenomena are measures of evolutionary progress with respect to ancestral affinities and reproductive isolation. In a general way fertility and viability have been found to decrease in interspecific hybridization as the parental forms are progressively more remotely related. Many other measures can be used for such an assay; some of which, including genetical and cytogenetical characteristics, have already been reported (Danforth and Sandnes, 1939; Danforth, 1950; Sandnes, 1954), while additional information including sex-ratios and the genetical and physiological basis of sterility and intersexuality, will form the subject of future contributions. Some clarification may be desirable in regard to the use of the term "sterility." In this series of birds the most pronounced examples of sterility are those dependent on apparently complete absence of any germinal epithelium. Equally sterile from a functional point of view, but with a slight advance in the direction of fertility, are other specimens in which the germinal epithelium does contain, at least during the embryonic ages, recognizable germ cells which do not develop further. Still less pronounced cases of sterility show well-developed gonads which apparently never produced any mature germ cells or only germ cells which proved to be non-functional. Even individuals with seemingly normal gonads and germ-cell production may give rise to no zygotes at all, to zygotes which fail to develop, or to embryos and young which die prematurely. The final stages are represented by those seemingly normal individuals which present a graded series from what amounts to complete sterility to a high degree of fertility. Thus, fertility may be regarded as the converse of sterility, but it is measurable over only a segment of the sterility scale. The fertility in both sexes is here expressed as percentage of fertile eggs produced. The highest degree of reproductive compatibility (converse of reproductive isolation, Dobzhansky, 1941) is represented by the production of viable hybrids which in turn are fully fertile. While the present report deals more specifically with fertility data as such it may nevertheless be appropriate to mention briefly the degree of sterility encountered in the hybrids from some of the crosses. In a later contribution it will be shown that meiosis in Phasianus-Gallus hybrids rarely proceeds beyond the stage of chromosome pairing in the males, and never beyond that stage in females. Gonad development is also extremely retarded in other respects, especially in females where some actual regression occurs, and general hormone secretion is lacking or extremely slight as evidenced by the status of accessory and secondary sex traits. A few individuals showed evidence of sex inversion. Similarly, females from the F1 and the first backcross, as well as some from other crosses involving Phasianus and Chrysolophus, show marked sterility and intersexuality. Strong evidence for sex reversal in some hybrids derived from these genera will be adduced. Viability is measured in terms of dura-