The locus estimated by the BC data at 19 is probably closer than the one at 4. We hope to obtain a closer estimate from 3-point TC data.
Dept. Veg Crops, Univ. Calif. Davis, CA 95616
Multiple cotyledons has been the subject of much interest in tomato genetics for a long time. George Reynard investigated this trait as part of this PhD thesis (TGC 2:7-8, 1952). In his materials the polycot trait was inherited quantitatively; homozygous multicot lines were not derived. We obtained our material from Archie Millett, who discovered the trait segregating in a breeding line. The original line and the present TGRC stock (LA2896) is in a determinate stock, a factor that might affect expression of certain morphs.
Polycot is a highly pleiotropic mutant. Cotyledon expression is variable but not confused with normal type. In homozygous progenies tricots are most abundant; many tetracots are seen, rarely seedlings with more than 4 cots; the remainder have two cotyledonary elements but variously bifid or exceptionally broad with two midveins. True leaves are also clearly abnormal; smaller with fewer segments than wildtype, entire margins, grayish, strongly convex, and deep-veined. The plants are markedly stunted but grow reasonably well in the field. Phyllotaxy is normal. Inflorescence are large, compound, and exinastic; in fact, over a long growing season the plant becomes covered by a tangle of inflorescence, obscuring the foliage. Flowers tend to be fascinated, variously malformed, and corolla segments are narrow. Few fruits are set spontaneously, but hand selfing results in a good set of subnormal, variably deformed fruits, usually with good seed yields.
Crosses with wildtype yield normal F1's. F2's and testcrosses segregate in monogenic fashion, although pct's tend to be variably deficient in F2. The high heterogeneity might relate to genetic differences between the wildtype parents or, more likely, to variable germination problems. Data accumulated thus far:
________________________________________________ Generation + pct X^2^ ________________________________________________ F2 133 49 0.36 F2 115 22 5.84* F2 158 9 34.24*** F2 180 32 11.09*** F2 132 32 2.63 F2 108 38 0.08 F2 168 51 0.34 F2 281 56 12.63 TC 30 42 2.00 ________________________________________________ Total F2 1275 289 34.78*** Heterogeneity 23.43*** ________________________________________________The single testcross was made with LA716 (L. pennellii); the excess of pct is typical of the favoring of recurrent alleles in this cross.
Various crosses were made to search the linkage relations of pct. After a protracted chase around the genome, pct was crossed with LA716 and the testcross assessed for allozyme segregation. The data revealed a significant linkage dissociation between pct and EST-2p: 10 + +: 26 pct +: 20 + +/Est-2p: 14 pct +/Est-2p; X^2^indep.=23.1***. Estimated pct Est-2 recombination = 19 cM.
Following this lead, we hybridized pct with LA983 (which chromosome 9 markers ah marm). the F1 had normal phenotype and yielded F2 populations with results summarized as follows:
160 + + + 50 pct + + 38 + + marm 4 pct + marm 48 + ah + 1 pct ah + 35 + ah marm 1 pct ah marmA highly significant repulsion between pct and ah (X^2^ = 16.5***) is revealed with an estimated distance of 20 cM; a lesser repulsion between pct and marm (X^2^ = 6.0*), est. linkage = 33 cM: and the expected association between ah and marm, highly significant (X^2^ = 22.4***), est. linkage = 33 cM. The recovery of a triple recessive homozygote rules out a locus between ah and marm, and the much tighter linkage with ah than with marm indicates that pct resides on the short arm. The aforementioned pct--Est-2 relations are compatible with this site. Also helpful in the recovery of this recombinant is the potential of deriving progeny to provide a useful new 3-point seedling linkage marker stock for chromosome 9. These data indicate the following map relations:
The locus estimated by the BC data at 19 is probably closer than the one at 4.
We hope to obtain a closer estimate from 3-point TC data.