Reproduction and the origin of polyploids in hybrid salamanders of the genus Ambystoma

Eggs and larvae produced by diploid, triploid, and tetraploid females collected from breeding ponds on Pelee Island in Lake Erie were studied to examine the reproductive mechanism. No instance of parthenogenesis was found as all examined females required sperm to produce viable progeny. Diploid females produced diploid and triploid larvae, triploid females produced triploid and tetraploid larvae, and tetraploid females produced triploid and tetraploid larvae. The majority of the eggs produced by hybrid females do not develop or do not complete embryogenesis. Electrophoretic examination of females and their offspring demonstrate that the male genome is being incorporated in reduced as well as unreduced eggs produced by all three ploidy classes of females. The elevation of ploidy among Pelee Island Ambystoma is attributed to sperm incorporation in unreduced eggs. Triploid as well as tetraploid individuals are constantly being produced. A critical examination of the literature on parthenogenetic or gynogenetic modes of reproduction in North American Ambystoma hybrids shows no conclusive evidence supporting these modes and it is suggested that the reproductive mechanism found among Pelee Island female hybrids may be more generally applied to other hybrid Ambystoma populations.Key words: Ambystoma, polyploids, hybrids, electrophoresis, reproduction.

TRANSFER OF LEAF RUST RESISTANCE FROM AEGILOPS SPELTOIDES TO TRITICUM AESTIVUM

Five accessions of Aegilops speltoides resistant to race 5 of leaf rust were crossed and backcrossed four or five times to wheat cultivars Manitou and Neepawa. The resistance transferred from each accession to the recurrent parents appeared to be controlled by one incompletely dominant gene. Since a majority of resistant lines show complete chromosome pairing when heterozygous and segregate in a monofactorial ratio of 3 resistant to 1 susceptible, the Aegilops genes must be on wheat chromosomes. The present experiment shows that if extensive homoeologous chromosome pairing occurs in the F1 hybrids, introgression can occur directly from a diploid into a hexaploid. The origin of polyploids in the Triticum-Aegilops polyploid complex is briefly discussed, and it is stressed that polyploidy is closely and inversely related to introgression. Whether introgression or polyploidy is going to occur depends largely on the genomic relationships and the extent of chromosome pairing in the F1 hybrid.