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  • Author or Editor: Bruce R. Carle x
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The recessive leaf trait, fused vein (fv), in Cucurbita pepo L. is expressed by the sixth leaf stage and then throughout vegetative growth. It is characterized by the partial fusion of the lateral leaf veins to the main central vein. Consequently, the dorsal leaf surface is distinctively puckered. Use of fv as a genetic marker in hull-less seeded pumpkin lines is hampered, however, by a low recovery of fv plants in segregating populations. Homogeneity Chi Square analysis of 11 F2 (3:1 X2 = 72.05 P < 0.005) and 16 BC (1:1 X2 = 120.12 P < 0.005) populations indicated significant heterogeneity between populations for fv recovery. Recovery ranged from 0 to 35.5% for 11 F2 populations and from 6.8 to 75.4% for 16 BC populations. There was a significant reduction, 35%, in seed yield/fruit when fv pollen was used to hand pollinate fv, normal (N), and F1 flowers as compared to pollinations with N pollen. In pollen competition studies, reduced competition at low levels of F1 or 50:50 fv/N pollen increased fv recovery in F2 and BC populations. These results are consistent with the hypothesis that the fv trait confers gametic subvitality resulting in distorted Mendelian segregation.

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Three experiments were conducted to delineate gametophytic selection of the fused vein trait in Cucurbita pepo L. Gametophytic subvitality was verified by comparing fused vein and normal pollen tube growth. Microscopic examination of partitioned, co-pollinated flowers revealed fewer and slower growing fused vein tubes than normal. The effects of gametophytic subvitality on seed yield and inheritance were shown by manipulating the severity of reproductive competition. Fused vein, normal, and F1 lines were pollinated with fused vein, normal, 50:50 mix, and F1 pollen at three different pollen loads. Analysis showed that fused vein pollen generated significantly fewer seed per fruit in all lines. In ensuing F2 and testcross populations, a reduction in load and thus competition significantly increased the number of fused vein individuals. Leaf number and area for normal, fused vein, F1, F2, and testcross plants were assessed to test pleiotropic effects on growth common to gametophytic subvitals. Although normal and fused vein lines differed in leaf number and size, their total leaf areas were not significantly different. F2 and testcross plants showed no difference between normal and fused vein individuals; leaf size and number were independent of leaf morphology.

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Genetic experiments were initiated to assess the potential for combining large seed size from PI 285611, a large-fruited, hullless seeded accession, with small fruit size from a hullless seeded breeding line (NH29-13-5-4). An F2 population and parental line were field-grown during Summer 1993 to determine inheritance and heritability of large seed size and the relation between fruit and seed size. Seed size variables of weight, width, length, and thickness were regressed against fruit weight. There was a moderate, positive correlation between large fruit and seed length (R2 = 0.46). However, seed thickness, a major determinant of seed weight, was not correlated with fruit size. In an F2 population of ≈450 plants, there was a small number of plant selections with fruit under 1.5 kg and seed size approaching that of PI 285611.

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Two experiments were conducted to test and delineate gametophytic subvitality of the fused vein trait in Cucurbita pepo. Gametophytic subvitality was verified by comparing pollen tube growth for fused vein and normal pollen in situ. Microscopic examination of partitioned, co-pollinated distillate flowers revealed inferior fused vein gametophyte performance. Normal pollen tubes grew faster and were significantly more abundant in the lower portion of the style. The consequences of gametophytic subvitality on seed yield and inheritance were shown by manipulating the severity of pollen competition. Fused vein, normal and F1 lines were pollinated with fused vein, normal, F and a 50:50 pollen mix at three different pollen loads. Fused vein pollen generated significantly fewer seed per fruit in all female genotypes. As a constituent in F, or mixed pollen, it produced significant seed yield reductions at the low pollen load. In F1 and testcross populations, a reduction in pollen load and therefore pollen competition significantly increased the number of fused vein individuals in segregating populations.

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The morphology, growth rate and anatomy of the fused vein trait were characterized in Cucurbita pepo using the inbreds NH2405 (fused vein), NH7210 (moderately fused vein), and NH614 (normal). Morphological analysis showed that the trait is characterized by a partial fusion of the five primary leaf veins. Fusion begins at the distal point of the petiole and extends along the central vein. Branching of the veins is delayed and there is a reduction of the interveinal leaf blade. Consequently, the upper leaf surface appears puckered or wrinkled. Depending on genetic background, the onset of fused vein leaf production starts at the fourth to tenth leaf stage and continues throughout vegetative growth. The extent of fusion increases with leaf number but stabilizes by the twentieth leaf stage maximum extent of vein fusion also varies with genetic background (5-20 cm). Though fused vein and normal inbreds differed in the rate and pattern of leaf growth, examination of F2 and BC populations revealed no significant effect of the fused vein trait on leaf number, leaf size, and rate of leaf initiation. Anatomical examination revealed different vascular patterns in the transition zone between petiole and leaf blade for normal and fused vein leaves. In normal leaves, the vascular bundles of the petiole enlarge and coalesce to form a vascular crescent. The crescent reorganizes and diverges as large vascular columns and pairs of smaller flanking vascular bundles into each vein. In contrast, two cycles of enlargement, coalescence, and dispersal occur in fused vein leaves.

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The expression and inheritance of the fused vein trait in Cucurbita pepo were investigated. The fused vein inbred, NH2405, was crossed to normal lines, NH614, and NHBP10. Reciprocal F1, F2, F3 and BC populations were generated and examined for leaf type segregation in field and greenhouse environments. Although the fused vein phenotype is stable in NH2405, it exhibited a continuum of expression in segregating populations. The onset of vein fusion ranged from the fourth to the tenth leaf stage and degree of fusion varied from slight (1-5 cm) to extreme (10-20 cm). Inheritance ratios varied with population, conditions of production, and direction of cross. Most segregating populations fit either a single or double recessive gene model, however, a quarter of the populations showed no or low fused vein recovery. A feasible explanation for the distorted inheritance is that the fused vein trait is a gametophytic subvital, governed by a single recessive gene, fv. Although less likely, a double recessive, subvital model cannot be ruled out.

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A joint breeding effort of the Universities of Puerto Rico and Florida involves the development of short-vined tropical pumpkin (Cucurbita moschata) genotypes that are able to reach maturity earlier than traditional long-vined types. Sixteen promising hybrids and inbreds were planted in Lajas, Puerto Rico, in June 1998. Pedigrees of this material included traditional tropical genotypes crossed with bush or compact temperate genotypes. Anthesis in both pistillate and staminate flowers occurred on average 49 days after planting. However, the variability of flowering dates among genotypes was far greater for pistillate (40 to 60 days) than staminate (46 to 54 days) flowers. Hybrids flowered earlier than inbred lines. Female flowers opened before male flowers in many genotypes. It seems likely that an inadequate source of pollen contributed to the low yields of some of the earliest genotypes. The five highest-yielding genotypes had pistillate flowers that opened after their male counterparts. All plots were once-over harvested 86 days after planting. Average yield per plant varied from 1.4 to 6.0 kg. Average fruit weight varied from 0.8 to 3.1 kg. High-yielding genotypes tended to have the highest fruit weight, a factor that should be considered when breeding for the next generation of short-vined genotypes. Yields were less than what could be expected from a long-vined tropical pumpkin. However, this yield could be obtained with a once-over harvest at about 90 days, compared to multiple harvests beginning at 120 days, saving costs of additional field practices, and allowing the land to be used for other purposes.

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A new leaf mutant, fused-vein, is described in Cucurbita pepo L. for use as a plant gene marker. Morphologically, the fused-vein trait is characterized by a partial fusion of the lateral leaf veins to the main central vein. Fusion begins at the distal point of the petiole and extends for 5 to 10 cm into the leaf blade, thereby delaying branching of the leaf veins and causing the dorsal leaf surface to appear puckered. The trait is expressed beginning at the fourth to sixth leaf stage and throughout vegetative growth. Preliminary inheritance data suggests a two gene, double recessive model. Data on segregation of the fused-vein trait in reciprocal F2 and backcross progenies will be obtained during the summer of 1990. This trait has been incorporated into hull-less seeded lines for use as a marker to identify rogue genotypes. Comparison of F5 sister lines with and without the fused-vein trait indicated that the trait does not affect fruit or seed yields.

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A genetic linkage [randomly amplified polymorphic DNA (RAPD)-based] map was constructed for watermelon [Citrullus lanatus (Thunb.) Matsum and Nakai] using a BC1 population [PI 296341-fusarium wilt resistant × New Hampshire Midget (fusarium susceptible)] × `New Hampshire Midget'. The map contains 155 RAPD markers, and a 700-base pair sequenced characterized amplified region (SCAR) marker that corresponds to a fragment produced by the RAPD primer GTAGCACTCC. This marker was reported previously as linked (1.6 cM) to race 1 fusarium wilt resistance in watermelon. The markers segregated to 17 linkage groups. Of these, 10 groups included nine to 19 markers, and seven groups included two to four markers. The map covers a genetic linkage distance of 1295 cM. Nine of the 10 large linkage groups contained segments with low (or no) level of recombination (0 to 2.6 cM) among markers, indicating that the watermelon genome may contain large chromosomal regions that are deficient in recombination events. The map should be useful for identification of markers linked closely to genes that control fruit quality and fusarium wilt (races 1 and 2) resistance in watermelon.

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