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Breeding Behavior of Maize-Tripsacum Hybrids¹

Hybrids bewteen Zea mays L. and octaploid Tripsacum dactyloides (L.) L. are male sterile but female fertile. They reproduce by means of aposporous gametophytic apomixis, or sexually. The 36 Tripsacum chromosomes of these hybrids pair preferentially, while the 10 Zea chromosomes remain as univalents during meiotic prophase and are often excluded from the daughter nuclei. Failure of cytokinesis during the second meiotic division can then give rise to gametes with 36 Tripsacum chromosomes only. These may be fertilized when the hybrids are backcrossed with maize as the pollen parent to give offspring that resemble the parental hybrids in genome constitution. Such sexual gametes apparently never develop parthenogenetically. Occasional pairing between one to four Zea and Tripsacum chromosomes makes intergenome segmental exchange possible. This results in later backcrosses to regular inclusion of Zea chromosomes in the daughter nuclei at the first meiotic division, and functional female gametes with 36 Tripsacum and 2, 4, 6, or 8 Zea chromosomes in homologous pairs are produced.

Key Words: Zea mays • Tripsacum dactyloides • Chromosome behavior • Apomixis • Corn

² Professors and graduate assistants, respectively, Crop Evolution Laboratory, University of Illinois, Urbana.

Received for publication September 27, 1972.