Cucurbit Genetics Cooperative Report 10:85-86 (article 45) 1987

Novel Variation in an Interspecific Cross

R. W. Robinson

Horticultural Sciences Department, New York State Agricultural Experiment Station, Geneva, NY 14456

An unusually large number of different chlorophyll deficient mutants was observed in breeding lines derived from the interspecific cross, Cucurbita maxima x C. ecuadorensis. One F₂ population segregated 147 normal : 18 albino seedlings that died in the cotyledon stage. Eight other F₂ populations, derived from different F₁ plants, did not segregate for the albino mutant. When the F₁ plant that produced albino progeny was backcrossed, to different plants of both species than used in the original cross, none of the backcross plants was albino. Thus, the albino mutant is recessive and probably due to a single gene with disturbed segregation ratio. It could have been present in heterozygous form in only one of the parental plants, or it may have occurred as a spontaneous mutation in a gamete of one of the parental plants and was therefore transmitted to only one of the F₁, plants.

Other chlorophyll deficiency mutants occurred, but not until subsequent generations of the same interspecific cross. Eleven advanced breeding lines segregated for seedlings with chlorotic cotyledons. Each of the 11 lines was derived from a different F₂, plant with normal phenotype, and the mutants are therefore probably the result of 11 different mutations. Mutants of one of the lines had cream colored cotyledons, and mutants of the other lines had cotyledons of varying degrees of yellow or light green. Each of the 11 mutants is recessive and probably monogenic. Mutants of one of the lines were lethal in the seedling stage. The others survived, although they remained chlorotic; they were fertile and produced seed under field conditions and are useful as seedling marker genes.

Cutler and Whitaker (1) also found novel variation in progeny of this interspecific cross. They reported finding various patterns of chlorophyll deficiency in F₂ and BC generations of C. ecuadorensis x C. maxima. Wall and Whitaker (4) reported F₂ segregation of 3:1 for one of these mutants, which had chlorotic leaves, stems, and petioles.

The disturbed segregation ratio of the albino mutant that segregated in one of the F₂ populations is not unique. Weeden and Robinson (5) reported significant deviations from Mendelian segregation ratios for 14% of the data for allozyme segregation in progeny of C. maxima x C. ecuadorensis.

Novel variation has been reported previously in progeny of species hybrids. Rick (2) concluded that the most likely source of the unusual variants he found in progeny of interspecific Lycopersicon crosses was heterozygosity in the self incompatible, wild species used as parents. Cucurbita ecuadorensis is self compatible and thus less likely to accumulate heterozygotes for deleterious, recessive genes than are the obligate outcrosssing Lycopersicon species. No chlorophyll deficient or other mutants were found in the self pollinated progeny of several C. ecuadorensis plants, indicating that heterozygosity for deleterious recessive genes is not common in that species. The original albino mutant found in the F₂ of C. maxima x C. ecuadorensis could have resulted from heterozygosity of one of the parental plants, but the 11 other mutants found subsequently have a different origin since none of these 11 mutants segregated in the F₂, generation of the interspecific cross. It wasn't until one or more additional generations of pedigree selection, or backcrossing to C. maxima followed by selfing, that the mutants were observed. Genome-cytoplasm interaction or complementary interaction of genes of the two parental species are also unlikely causes for the 11 mutants, since they did not occur in the F₂ of the interspecific cross. The relatively large number of different mutants found is probably not due to chance, and may reflect a high rate of mutation induced by hybridization such as that suggested by Sturtevant (3).

Literature Cited

1. Cutler, H. C. and T. W. Whitaker. 1969. A new species of Cucurbita from Ecuador. Ann. Mo. Bot. Gdn. 55:392-396.
2. Rick, C. M. and P. G. Smith. 1953. Novel variation in tomato species hybrids. Amer. Nat. 87:359-373.
3. Sturtevant, A. H. 1939. High mutation frequency induced by hybridization. Proc. Natl. Acad. Sci. 25:308-310.
4. Wall, J. R. and T. W. Whitaker. 1971. Genetic control of leucine aminopeptidase and esterase in the interspecific cross Cucurbita ecuadorensis x C. maxima. Biochem. Genet. 5:223-229.
5. Weeden, N. F. and R. W. Robinson. 1986. Allozyme segregation ratios in the interspecific cross Cucurbita maxima x C. ecuadorensis suggest that hybrid breakdown is not caused by minor alterations in chromosome structure. Genetics 114:593-609.