In order to generate new varieties, this study focused on the rescue and use of landraces and wild
The origin of papaya (
One of the problems affecting this fruit crop is the reduced number of commercial varieties exploited [
In the present work, the resulting F1 and F2 populations from the crosses between the local parent receiver line 7 and the parent donor M22 (L7 × M22) were evaluated using a series of morphological traits in order to identify and select new material to include in the ongoing breeding program seeking new varieties that show both high number of fruits and adequate plant height.
The present work was conducted at the Scientific Research Center of Yucatan (CICY). First, the receiver parent line 7 (L7; hermaphrodite; yellow-fleshed fruits) was crossed with the pollen donor parent Maradol 22 (M22; hermaphrodite; orange-red fleshed fruits).
From one of the fruits resulting from these crosses, 48 seeds were germinated, and the resulting plants were grown until they produced fruits (F1). From those, only 3 individuals were selected and again the flowers were bagged to ensure no contamination with foreign pollen while allowing self-pollination to continue with the F2 characterization (26 genotypes). The seeds from the resulting progeny were propagated in nursery trays under greenhouse conditions for 2 months. They were then transferred to the field where they were managed under commercial fertilization, irrigation, and pest-control conditions, until fruit setting 14 months later.
The resulting plantlets showed a sexual type segregation of 28 hermaphrodites and 14 female plants at F1 and 18 hermaphrodites and 8 female plants at F2. Regarding flesh color segregation, the F1 showed 15 yellowed-fleshed fruits and 13 orange-red fleshed fruits, while the F2 showed 1 yellowed-fleshed fruits and 25 red orange-fleshed fruits.
Those hermaphrodite plantlets (28 plants F1 and 15 plants F2) were subjected to further characterization and evaluation of morphoagronomical characters to select superior individuals. The data recorded were plant height (PH), height of first fruit (HFF), stem diameter (SD), petiole length (PL), number of flowers per node (NFLN), number of fruits per node (NFN), and number of fruits per plant (NFP). The selection was made based on international descriptors as reported by [
Phenotypic variation and distribution within the F1 and F2 populations derived from the cross L7 × M22 are continuous and normal (Figure
Statistics parameters from values of seven morphological traits measured in the parent lines L7 and M22 as well as in the F1 and F2 progenies, resulting from the intraspecific cross L7 × M
Parameter | PH | HFF | SD | PL | NFLN | NFN | NFP |
---|---|---|---|---|---|---|---|
M22 | 165.2 | 85.0 | 12.2 | 72.0 | 4.0 | 1.8 | 23.8 |
L7 | 203.0 | 98.0 | 11.5 | 89.0 | 6.0 | 4.0 | 68.0 |
Mean F1 | 195.5 | 94.8 | 9.36 | 84.1 | 4.2 | 1.3 | 29.5 |
Mean F2 | 229.6 | 115.0 | 11.1 | 76.4 | 2.6 | 2.2 | 26.6 |
Minimum F1 | 164.5 | 47.8 | 6.8 | 50.8 | 3.0 | 1.0 | 6.0 |
Minimum F2 | 170.0 | 62.0 | 6.2 | 50.0 | 1.0 | 1.0 | 6.0 |
Maximum F1 | 238.0 | 120.5 | 11.9 | 11.8 | 7.7 | 2.3 | 67.5 |
Maximum F2 | 297.0 | 187.0 | 18.0 | 119.0 | 7.0 | 5.0 | 94.0 |
S2 F1 | 18.9 | 15.3 | 1.29 | 13.5 | 1.5 | 0.4 | 14.6 |
S2 F2 | 44.8 | 32.5 | 2.6 | 15.7 | 1.6 | 1.2 | 21.2 |
CV (%) F |
10.2 | 16.2 | 13.8 | 16.1 | 35.2 | 32.4 | 49.5 |
CV (%) F |
19.5 | 28.3 | 23.5 | 20.5 | 61.5 | 56.8 | 79.9 |
Normality* F |
0.9763 | 0.9434 | 0.9676 | 0.9835 | 0.7977 | 0.8243 | 0.9731 |
Normality* F |
0.8893 | 0.9548 | 0.9852 | 0.9487 | 0.8742 | 0.8333 | 0.7148 |
PH: plant height, HFF: height of first fruit, SD: stem diameter, PL: length of petiole, NFLN: number of flowers per node, NFN: number of fruits per node, NFP: number of fruits per plant, S2: standard deviation, CV: coefficient of variation, L7: parent receiver, M22: parent donor, and F1 and F2: F1 and F2 progenies; *Shapiro-Wilk test.
Normal distribution of data from PH, HFF, SD, and NFP measured in the F1 and F2 progenies, resulting from the intraspecific cross between L7 × M22.
Figure
Mean values data from PH, HFF, SD, and NFP measured in the F1 and F2 progenies, resulting from the intraspecific cross between L7 × M22. Data from both parent lines are also shown. Bars represent standard errors.
In relation to plant height (PH), 67.3% of the F1 population showed intermediate values between those shown by the parents, while the remaining 32.1% had higher values than that of the parent L7. However, in F2, only 33.3% of the population had intermediate PH values between those of the parents, while 66.6% showed PH values above those shown by the parent L7. The mean values of PH and HFF indicated some degree of reciprocity in both F1 and F2 progenies. Thus, HFF within the F1 progeny showed a distribution of 17.8%, 42.9%, and 39.3% which corresponds to values below those of the parent M22, intermediate values between both parents, and values above those of the parent L7, respectively. The F2 progeny showed 20%, 20%, and 60.0% which corresponds to values below that of the parent M22, intermediate value between both parents, and values above that of the parent L7, respectively.
In conclusion, up to 60.7% of the F1 progeny and only 40% of the F2 progeny showed desirable HFF values and favorable harvesting time. The F2 population showed higher proportion of tall plants and plants with few fruits, in such a way that it would be convenient to backcross them with the M22 parent or sibmate them to improve fruit productivity and plant height in the next generation.
L7 and M22 showed contrasting genetic basis in traits of interest such as NFP, PH, and HFF, consistent with the variability observed in the F1 and F2 progenies. Table
Correlation matrixes obtained from the comparison of seven morphological traits measured at F1 (a) and F2 (b) progenies, resulting from the intraspecific cross L7
Progeny F1
PH | HFF | SD | PL | NFLN | NFN | NFP | |
---|---|---|---|---|---|---|---|
PH | 1.0000 | ||||||
HFF | 0.3299 | 1.0000 | |||||
SD | 0.3527 | 0.2675 | 1.0000 | ||||
PL | 0.1097 | 0.1885 | 0.4705 | 1.0000 | |||
NFLN | 0.2100 | −0.2848 | 0.2989 | 0.3181 | 1.0000 | ||
NFN | 0.1926 | −0.3350 | 0.3651 | −0.0460 | 0.7178 | 1.0000 | |
NFP | 0.3473 | −0.1397 | 0.5309 | 0.1761 | 0.7347 | 0.7720 | 1.0000 |
Progeny F2
PH | HFF | SD | PL | NFLN | NFN | NFP | |
---|---|---|---|---|---|---|---|
PH | 1.0000 | ||||||
HFF | 0.4146 | 1.0000 | |||||
SD | 0.3837 | 0.4759 | 1.0000 | ||||
PL | 0.4795 | 0.4031 | 0.5055 | 1.0000 | |||
NFLN | 0.3557 | 0.4361 | 0.4603 | 0.4051 | 1.0000 | ||
NFN | 0.2728 | 0.3654 | 0.3295 | 0.3215 | 0.8622 | 1.0000 | |
NFP | 0.4116 | 0.2865 | 0.5690 | 0.3237 | 0.7877 | 0.8055 | 1.0000 |
PH: plant height, HFF: height of first fruit, SD: stem diameter, PL: length of petiole, NFLN: number of flowers per node, NFN: number of fruits per node, NFP: number of fruits per plant, and F1 and F2: F1 and F2 progenies.
The mean values of NFN and NFP at both F1 and F2 progenies were closer to those of M22 but lower when compared to the parent L7, indicating that the F1 and F2 populations had promising individuals to be selected on the search of genotypes showing high NFP.
The PH and HFF showed a variability of 10.2% and 16.2% in F1 progeny and 29.5% and 28.3% in the F2 progeny, respectively. In PH, the 32.14% of the F1 population and 66.6% of the F2 population exceeded the PH of the parent L7 that is an undesirable characteristic for the breeder, while 67.86% and 33.33% of the F1 and F2 progenies, respectively, showed intermediate plant height between those of L7 and M22 parents, which is useful for breeders seeking decreased plant height as indicated by Esquivel et al. [
SD had a variability of 13.8% to 23.5% in F1 and F2 progenies. 92.85% and 66.6% of the F1 and F2, respectively, showed a slightly lower SD mean value than that of the parents L7 and M22, while 7.15% of the F1 and 13.3% of F2 progeny were plants with intermediate SD between those of their parents, while only 20.0% of the F2 progeny exceeded the stem diameter of the parent M22. In this regard, [
Data from the parent lines, L7 and M22, as well as those from the F1 and F2 progenies, were subjected to cluster analysis by similarity or dissimilarity, according to the phenotypic trait evaluated. Four groups were obtained by cluster analysis in the F1 population (Table
Groups formed by cluster analysis from seven morphological traits in the progeny F1 (a) and progeny F2 (b) resulting from the intraspecific L7
Progeny F1
Group | PH | HFF | SD | PL | NFLN | NFN | NFP |
---|---|---|---|---|---|---|---|
1 | 201.56 | 110.37 | 9.38 | 69.97 | 3.06 | 1.07 | 19.49 |
2 | 174.94 | 80.30 | 9.00 | 65.81 | 3.77 | 1.20 | 24.52 |
3 | 210.59 | 92.27 | 9.24 | 79.88 | 5.42 | 1.43 | 40.88 |
4 | 198.08 | 89.29 | 11.25 | 76.69 | 6.92 | 2.75 | 62.87 |
Progeny F2
Group | PH | HFF | SD | PL | NFLN | NFN | NFP |
---|---|---|---|---|---|---|---|
1 | 193.44 | 85.80 | 9.62 | 64.20 | 2.00 | 1.36 | 19.16 |
2 | 240.78 | 115.11 | 11.27 | 76.59 | 2.46 | 2.03 | 20.96 |
3 | 203.00 | 98.00 | 11.50 | 89.00 | 6.00 | 4.00 | 68.00 |
4 | 288.00 | 172.50 | 14.95 | 103.50 | 5.65 | 4.45 | 79.00 |
PH: plant height, HFF: height of first fruit, SD: stem diameter, PL: length of petiole, NFLN: number of flowers per node, NFN: number of fruits per node, NFP: number of fruits per plant, and F1 and F2: F1 and F2 progenies.
Clustering dendrograms constructed using Ward’s method based on the squared Euclidean distances from analysis of seven traits from the F1 (a) and F2 (b) progenies resulting from the intraspecific cross between L7 × M22. The parent lines L7, parent receiver line 7, and M22, parent donor Maradol 22, are indicated, H6B to H78B and H90B: F1 progeny. A3 to C51: F2 progeny.
A second group was formed by H7B, H10B, H11B, H17B, H19B, H67B, H68B, H69B, H70B, and H90B. This group, together with the parent M22, exceeded in 20.5% the first group in terms of NFP, but it showed less PH and HFF. In a third group formed by H13B, H72B, H74B, H75B, H76B, and H77B, the F1 progeny had up to 40% more NFP than group two, associated with higher PH.
Finally, the fourth group was integrated by the receiver parent L7, together with H66B and H78B, having the highest value of NFP and SD, whereas the PH and HFF mean values indicated that the plants from this group had the lowest plant height.
The grouping of the F2 population is shown in Table
Notably, the SD mean value in the four groups of the F1 and F2 progenies was the highest within each population and showed a correlation with NFP of 0.5309 to 0.5690 in F1 and F2, respectively. Namely, plants that had higher SD had higher NFP. In this regard, [
This clustering analysis is useful first to screen genotypes with low or high productivity [
In order to identify traits that account for most of the observed variance, the data were subjected to principal component analysis. Table
Values and proportion of total variance explained by (a) a principal component analysis and (b) ratio of variability for each trait evaluated in the first three principal components obtained from the morphological analysis of seven traits at the F1 and F2 progenies resulting from the intraspecific cross L7
Principal component
Explained proportion of variance (%) | ||||||
---|---|---|---|---|---|---|
|
Eigen value | Absolute | Accumulated | |||
F1 | F2 | F1 | F2 | F1 | F2 | |
1 | 3.32 | 4.19 | 47.50 | 70.14 | 47.50 | 70.14 |
2 | 1.47 | 0.88 | 21.06 | 12.55 | 68.57 | 82.69 |
3 | 0.86 | 0.46 | 12.42 | 6.54 | 80.99 | 89.23 |
4 | 0.49 | 0.36 | 7.04 | 5.18 | 88.04 | 94.41 |
5 | 0.41 | 0.22 | 5.93 | 3.09 | 93.97 | 97.51 |
6 | 0.25 | 0.13 | 3.62 | 1.88 | 97.59 | 99.39 |
7 | 0.17 | 0.04 | 12.40 | 0.60 | 100.00 | 100.00 |
Trait
Principal component | ||||||
---|---|---|---|---|---|---|
|
PC1 | PC2 | PC3 | |||
F1 | F2 | F1 | F2 | F1 | F2 | |
PH | 0.5041 | 0.7521 | 0.6965 | 0.5026 | 0.3985 | 0.0229 |
HFF | −0.0628 | 0.7656 | 0.8654 | 0.3418 | −0.3354 | 0.4974 |
SD | 0.6509 | 0.8544 | 0.1763 | 0.1405 | −0.5939 | 0.2003 |
PL | 0.7503 | 0.8305 | 0.1928 | −0.3630 | −0.0975 | 0.0235 |
NFLN | 0.7923 | 0.8365 | −0.3214 | 0.2335 | 0.4503 | 0.3873 |
NFN | 0.8102 | 0.9304 | −0.2280 | −0.4944 | 0.0653 | −0.0318 |
NFP | 0.8925 | 0.8792 | −0.1309 | −0.2421 | −0.1702 | −0.1351 |
PH: plant height, HFF: height of first fruit, SD: stem diameter, PL: length of petiole, NFLN: number of flowers per node, NFN: number of fruits per node, NFP: number of fruits per plant, F1 and F2: F1 and F2 progenies, and PC: principal component.
The PC1 corresponding to F1 progeny showed a variance of 47.50% and the traits that explain the greater variability proportion are NFLN, NFN, and NFP, and the same principal component in the F2 explains 70.14% of the variance by the characters NFLN, NFN, NFP, and SD.
For PC2, a variance of 21.06% was obtained to characters PH and HFF in the F1 population, while for F2 progeny only the character PH represented a 12.55% variance.
PC3 in F1 progeny obtained a 12.42% variability and in F2 this was 6.54% represented by the SD and HFF, respectively (Tables
The distribution of individuals forming the F1 and F2 progenies, according to their similarities, is shown in bidimensional graphs; both were grouped according to the variability explained by PC1 and PC2 (Figure
Two-dimensional diagrams of the principal components 1 (PH, HFF) and 2 (NFLN, NFN, and NFP) obtained from the analysis of seven traits in the F1 (a) and F2 (b) progenies, resulting from the intraspecific cross between L7 × M22. Values from both parent lines, L7 and M22, are also indicated.
In the F1 population analysis, individuals were grouped in five groups. H66B and H78B were grouped in group one, and both individuals had the greatest value for NFP and SD, while values of HP and HFF grouped them as short plants. These short genotypes could be used in papaya breeding programs seeking reduced plant height as reported by Marin et al. [
The fourth group was formed by the individuals H8B, H9B, H12B, H14B, H15B, H16B, and H73B that showed low NFP and SD but also showed the highest HFF, parameters that classify them as unsuitable for selection. These undesirable traits are generally associated with longer internodes, widely spaced fruits, and shorter harvesting period [
The F2 population was also grouped into five groups. Group one included B15, B19, and L7, characterized by the largest NFP and PH, HFF, and SD, being the most productive and the tallest plants found in the F2 population. Group two was formed by A19, A46, B43, B52, C7, and C23, which had 65.4% less NFP and 22.3% less HFF than group one, although in PH both groups were very similar. Group three included A3, A45, and C16 that showed the lowest NFP of the F2 progeny; however, in PH, HFF, and SD, they showed a minimum reduction of only 8%, 2%, and 3%, respectively, from the values found in group two. Namely, plants from this third group are tall plants with very little fruit number. Group four included A35, C50, and C51, which showed a similar NFP to those from group three, but they had the lowest SD value within the F2 progeny. Likewise, group five composed of B35 and the parent M22 had the lowest PH and HFF within the F2 progeny but exceeded in 48% the NFP values found in the previous group.
Both clustering methods grouped genotypes based on the NFP trait that showed high variability in F1 and F2 progenies, followed by a dichotomic association with PH and HFF traits. The correlation of both sets of traits was useful to identify and group those individuals by their different proportion of productivity and plant height. Also in the F1 progeny, 75% of plants had low fruit productivity while 25% had high fruit productivity. In the F2 progeny, the proportion of low fruit productive plants increased to 87%, while that of plants with high fruit productivity decreased to 13%. Moreover, F1 progeny had a ratio of 55.0% of tall plants versus 45.0% of short plants, while in the F2 progeny, the proportion increased to 73% of tall plants and only 27% of short plants. The above-mentioned proportions are consistent with the expected allelic segregation, but both traits were influenced by multiple factors. The proportion of 3 : 1 for NFP in progenies F1 and F2 infers that they were influenced mainly by a heterozygous condition.
Niklas and Marler [
All breeding program seeking new varieties that meets the desirable characteristics requires characterization, evaluation, and selection of elite plants to be used to produce new crosses to continue with the ongoing breeding program. The F1 genotypes selected to obtain the F2 progeny were H10B, H11B, H17B, H19B, H66B, H67B, H68B, H69B, H70B, H77B, H78B, and H90B. Among the F2 plants, the genotypes selected were A3, A19, A35, A46, A45, B35 B43, B52, C7, C23, C50, and C51 as they met the desirable characteristics such as reduced plant height (PH), a condition that is important and desirable because it may thereby facilitate fruit harvesting. On the other hand, they also show intermediate number of fruits per plant (NFP) between those shown by both parents, which allowed the selection of papaya genotypes with improved productivity. These selected genotypes will provide the genetic base to generate new varieties which can compete with commercial varieties, as they should also show better adaptation to drought, heat, and perhaps diseases.
Cluster analysis and principal components analysis grouped F1 and F2 progenies derived from the cross L7 × M22 into less and more productive individuals and individuals with low and high plant height, and it can be used reliably as a tool in
Clustering analysis showed a ratio of 3 : 1 with respect to lower and higher NFP, and in F1 the ratio was 75.0% and 25.0%, while in F2 the ratio was 87.0% and 13.0%. In terms of plant height, it showed a ratio of 1 : 1 in the F1 progeny, with a proportion of 55.0% and 45.0%; however, a ratio of 3 : 1 was observed in the F2 progeny, corresponding to 27% and 73%. Namely, the F2 progeny showed the highest proportion of plants with less fruit and tall plants, so it is convenient to backcross it with the parent M22 to improve the number of fruits while maintaining a convenient plant height in the subsequent generations.
The present research will serve as a platform for an ongoing breeding program for genetic improvement of papaya in terms of NFP, PH, and HFF through the selection and integration of native materials with the characteristics of interest. The F1 genotypes selected for obtaining the F2 progeny were H10B, H11B, H17B, H19B, H66B, H67B, H68B, H69B, H70B, H77B, H78B, and H90B. Among the F2 plants, the genotypes selected were A3, A19, A35, A46, A45, B35 B43, B52, C7, C23, C50, and C51 to continue the ongoing papaya breeding process.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors wish to acknowledge CONACYT for the scholarship 35263 granted to Mariela Vázquez Calderón. The authors wish to thank Professor Raul F. Monforte-Peniche for kindly hosting part of their experiments at his property at Rancho San Pedro Sucilá, Yucatán, México, and Ing. Nelsy Pérez Monforte for providing support in the plantation management.