Graduate Thesis Or Dissertation
 

Embryo development in relation to interspecific hybridization of Phaseolus

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  • Interspecific hybridizations between P. vulgaris and P. acutifolius and between P. vulgaris and P. lunatus were investigated. Prefertilization events were similar in inter- and intra-specific crosses. Fertilization was completed in all crosses and the time of fertilization was dependent on the maternal parent, reflecting differences in the time of maturation of the female gametophyte. There were large reciprocal cross differences in the time of endosperm and embryo division. The time interval between fertilization and division of endosperm and embryo was longer than in selfings when species other than P. vulgaris were used as the female parent. In P. lunatus x P. vulgaris crosses, only 7-12% of the ovules contained dividing endosperms at 72 hours and the embryo ceased to grow at the four-celled stage. Embryos of P. vulgaris x P. lunatus developed up to the pre-heart shaped stage. Hybrid embryos between P. vulgaris and P. acutifolius reached the cotyledon stage, however, no mature hybrid seeds were obtained. The characteristic morphology of the interspecific hybrid embryos, uneven cotyledons, provided a clear distinction from selfed embryos. The growth rate and final size of hybrid embryos were influenced by the parental combinations. As hybrid embryos did not develop to maturity, embryo culture was used to recover hybrid plantlets. Glutamine was beneficial to the survival of hybrid embryos of P. vulgaris x P. lunatus. However, hybrid plants obtained were slow-growing and developed symptoms of premature senescence after three to four sets of trifoliolate leaves were formed. Embryos obtained from reciprocal crosses of P. vulgaris and P. acutifolius gave mature plants. Meiosis and fertility of interspecific hybrids between P. vulgaris and P. acutifolius were examined. At Metaphase I, the majority of the microsporocytes had 4 to 8 univalents with an average of 6 univalents per microsporocyte. However on the average, only 2 lagging chromosomes per cell were observed at Anaphase I. The most frequent chromosomal distribution at late Anaphase I was 10-12. It is likely that some univalents observed at Metaphase I may have resulted from precocious separation of loosely paired univalents. When these interspecific hybrids were self-pollinated, 26% of the ovules were fertilized; however, no dividing embryos were observed. When interspecific hybrids were used as the female parents in backcrossing to P. acutifolius and P. vulgaris, the frequencies of ovules being fertilized were 31 and 20% respectively, and the frequencies of ovules containing dividing embryos were 13 and 4%. The difference in the survival rate of backcrossed embryos suggested an influence of genomic dosage on embryo viability. Backcrossed-progeny were self-fertile and the seed set was two seeds per five pods. Additional experimental results and other observations provided indirect evidence that the development of interspecific hybrid embryos may be related to differences in the hormonal metabolism, possibly that of cytokinins, of parental species.
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