Differential Zygotic Lethality in a Tomato Species

PECIES hybrids are favorite materials for demonstrating a wide array of cytogenetic abnormalities, many of which can cause departures from normal segregation. Structural differences in the chromosomes with their familiar hereditary consequences are frequently encountered. Meiotic drive and preferential segregation phenomena are other examples of aberrancies that can profoundly affect genetic transmission. Lethality of gametes, independent of the foregoing causes, and selective fertilization by surviving gametes constitute additional mechanisms by which species hybrids can misbehave. Finally, lethality may also disturb segregation in postsyngamic stages. Simply inherited characters can be used to advantage in illuminating the nature of abnormal segregation of species hybrids. The requirements for such tests are: (1) sufficient fertility of the F, hybrids and their derivatives to permit the production of a reasonable number of progeny, and (2) monogenic markers of sufficiently strong phenotype that they can be distinguished in the typically highly heterogeneous background. Studies of this sort have been made in the progeny of species hybrids of Antirrhinum ( MATHER 1947), Gossypium (STEPHENS 1950), Melilotus (BAENZIGER and GREENSHIELDS 1958), and Zea (ROGERS 1950). In all instances, significant deviations from normal inheritance were observed, and in all investigations of backcross progenies except ROGER’S (1950) , differential elimination favored genes of the recurrent parent. The 12 instances of deviating ratios in his material [maize x F, (maize x teosinte)] favored genes of the recurrent parent in five, and those of the nonrecurrent parent in seven. The deviations were attributed by the respective writers to various presyngamic mechanisms of the foregoing list, but only STEPHENS (1950) argued in favor of a single cause of the deviations in his material: cryptic structural hybridity. The genus Lycopersicon is well suited for such investigations because its species can be hybridized in many different combinations, the Fl’s are reasonably fertile, a wide array of monogenic markers is known for L. esculentum, and the meiosis of F, hybrids is not complicated by differences in number or gross structure of chromosomes. Whereas segregations are generally normal in crosses between the closely related species (e.g. L. esculentum X L. pimpinellifolium) , they tend to be disturbed in wider crosses (e.g. L. esculentum x L. chileme). Observations on