الفهرس 
REVIEW OF LITERATURE
HeterosisOnly 14 pages are availabe for public view
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Abstract
The main objectives of this investigation were to determine the amounts of heterosis over both the mid-parents and the better parent, inbreeding depression, genetic parameters, nature of gene action, heritability in both broad and narrow senses for all traits under study on squash (Cucurbita pepo, L.) varieties.
The genetic materials used in the present investigation included five parental varieties of squash. These parental varieties were: Eskandarani (P1), Bianco di sicilia (P2), Zucchino striato d’Italia (P3), Zucchino mezza lungo bianco (P4) and Libanese phoma (P5). All these parental varieties representing a wide range of variability for most studied traits.
In the summer season of 1999, the parental varieties were crossed according to a complete diallel crosses mating design to produce 10 F1 hybrids and 10 F1r¬ (reciprocal) hybrids in addition, the five parental varieties were also selfed to obtain enough seeds from each variety.
In the summer season of 2000, all the 25 genotypes which included the 10 F1 hybrids, 10 F1r (reciprocal) hybrids and the five parental varieties were cultivated. The F1 hybrids and F1r (reciprocal) hybrids were self pollinated to produce the seeds of the F2 and F2r (reciprocal) generations. Similarly, the five parental varieties were also selfed to produce sufficient seeds from each parental variety and crossed to produce 10 F1 hybrids and their10 F1r (reciprocal) hybrids.
Genetic materials which obtained from the previous growing seasons were evaluated in growing seasons of 2001, in a field trail. The experimental design used was the randomized complete blocks design with three replications. The genetic materials used in this investigation consisted of five parental varieties were crossed according to a complete diallel crosses mating design to produce 10 F1 hybrids, 10 F1 reciprocal hybrids, 10 F2 generations and their F2 reciprocal generations.
The data were recorded for the following traits:
I. Vegetative and flowering traits:
1- Vein length (V.L. cm)
2- Number of leaves per plant (No.L./P.)
3- Number of the first female flowering node (No.1st.F.F.N.)
4- Date of the first female flower (D.1st. F.F.)
5- Date of the first male flower (D.1st.M.F.)
6- Number of male flowers per plant (No.M.F./P.)
II. Yield and yield component traits:
1- First picking date (1st.P.D.)
2- Number of fruits per plot (N.F./Pt.)
3- Fruit yield per plot (F.Y./Pt. kg)
4- Total weight of fruits/plant (T.W.F./P. kg)
5- Fruit length in centimeters (F.L. cm)
6- Fruit diameter in centimeters (F.D. cm)
7- Fruit shape index (F.Sh.I.)
8- Fruit weight in grams (F.W.g)
The obtained resulted could be summarized in the following points:
1- Tests of significance of the mean squares of the 45 genotypes which included five parental varieties and their F1, F1r (reciprocal) hybrids, their F2 and F2r (reciprocal) generations were highly significant for vegetative, flowering traits, yield and its component traits. These findings suggested that the planned comparisons could be valid. These results were expected where the studied genotypes in this investigation included variable genetic materials.
2- The results of means value show that no specific parent was superior for all studied traits. Generally (P1) was the lowest for D.1st F.F. (earlier) and No.M.F./P. On the other hand it was the highest for No.F./Pt., Y./Pt. kg (desirable) and F.Y./P. kg. The results also observed that (P3) was the lowest for V.L. cm, No.L./P. and No. 1st F.F.N. In the same time, it was the highest parent for D.1st.F.F., D.1st M.F., 1st.P.D., F.L. cm and F.Sh.I.
3- The performance of most F1, F1r (reciprocal) hybrids were variable and the results cleared that there is no hybrid gave the best results for all studied traits. While, most of F1 hybrids had the highest means of most studied traits compared with the parental inbred lines. The highest F1 hybrid was (P2 x P4) with the mean of 42.067 whereas, the highest F1r (reciprocal) hybrid was P5 x P3 with the mean of (42.6) for D.1st.F.F trait.
4- Concerning, the means of F2 generations compared with the F1 hybrids, the data cleared the presence of differences and the majority of F1 hybrids were higher than F2 generations.
5- The results of estimated amount of heterosis over mid-parents (M.P.) indicated that the highly significant heterosis values over mid-parents for all studied traits except for F.W (g) and ranged from - 9.43 to 39.844 % for F.D. (cm) and No. M.F./P. for F1 hybrids. While versus F1r (reciprocal) hybrids the values ranged from - 6.959 to 55.848 % for D.1st.F.F and No. M.F./P. In the case of heterosis over mid-parents versus (F1, 1r) hybrids the estimates ranged from - 6.76 to 47.83 % for the previous traits, respectively.
6- The estimates of heterosis versus better parent showed highly significant for most studied traits. The values of H (B.P., F1) ranged from – 23.64 % (F.D. cm) to 83.19 % (No.M.F./P.), heterosis H (B.P., F1r) ranged from - 19.32 % (F.D. cm) to 104.15 % (No.M.F./P.). Concerning heterosis H (B.P., F1, 1r) ranged from - 21.36% (F.D. cm) to 93.67 % (No.M.F./P.).
7- Means of F2 and F2r (reciprocal) generations were always lower than those F1 hybrids (F1) and F1 reciprocal (F1r) hybrids except for earleness traits. However, inbreeding depression were obtained.
8- The obtained results also indicated the presence of inbreeding depression for most studied traits I.D. (F1, F2), I.D. (F1r, F2r) and I.D. (F1, 1r, F2, 2r). The estimated values were greater for these traits which had large estimates of heterosis. The performances of all F2 generations (F2, 2r) were better than the parental varieties not only for vegetative traits but also for yield and yield component traits.
9- Estimation of genetic parameters showed that the non-additive genetic variance including dominance (2 D) were most important source of genetic variance. Additive genetic variance (2 A) also were very important not because the non-additive genetic variance including not only dominance but also epistasis. Reciprocal effects (2 r) were presented for all studied traits, these estimates were smaller than the additive (2 A) and non-additive genetic variance including dominance (2 D).
10- The results showed the importance of general (GCA) and specific (SCA) combining abilities. General combining ability (GCA) were larger than their corresponding estimates of specific combining ability (SCA) for all studied traits at both F1,1r hybrids and F2, 2r generations. Reciprocal effects (r) were significant for most studied traits.
11- Heritability values which were obtained for certain traits showed that heritabilty in broad sense (H2b %) were larger than the narrow sense heritabilty (H2n %). The estimated values of hertability in narrow sense (H2n %) were ranged from 0.00 to 60.38 % for 1st P.D. and F.Sh.I. from F1 hybrids and ranges from 0.00 to 68.019 % for No. M.F./P. and No.L./P. from F2 generations. The highest estimated values of heritability in broad sense (H2b %) were 85.377 % in the F1 hybrids for V.L. (cm) and 94.059 % in the F2 generations for the same trait.