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).
- Cutler, H. C. and T. W. Whitaker. 1969. A new species of Cucurbita
from Ecuador. Ann. Mo. Bot. Gdn. 55:392-396.
- Rick, C. M. and P. G. Smith. 1953. Novel variation in tomato species
hybrids. Amer. Nat. 87:359-373.
- Sturtevant, A. H. 1939. High mutation frequency induced by hybridization. Proc. Natl. Acad. Sci. 25:308-310.
- 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.
- 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.