Genetic parameters of resistance to Meloidogyne incognita in melon

Parâmetros genéticos da resistência a Meloidogyne incognita em meloeiro

ABSTRACT:

In order to assess the genetic control of resistance in the melon ‘Gaúcho Redondo’ to the root-knot nematode Meloidogyne incognita, an experiment was conducted in a randomized complete block design with three blocks and six treatments using the parental lines ‘Gaúcho Redondo’ (P₁ resistant) and JAB 20 (P₂ susceptible), as well as F₁, F₂, and backcross generations (RC₁P₁ and RC₁P₂). Seventy days after inoculation, individual plants were evaluated for resistance using the nematode reproduction factor (RF). The hypothesis of monogenic inheritance was rejected by the chi-square test (χ²), and results indicated that resistance is controlled by more than one gene locus, as confirmed by the quantitative analysis that revealed the presence of six genes.

Key words:
Cucumis melo; genetic inheritance; plant breeding; reproduction factor

RESUMO:

Com o objetivo de avaliar o controle genético da resistência do melão ‘Gaúcho Redondo’ ao nematoide de galha Meloidogyne incognita, foi conduzido um experimento em blocos casualizados com três blocos e seis tratamentos, os quais envolveram as linhas parentais ‘Gaúcho Redondo’ (P₁, resistente) e JAB 20 (P₂, suscetível), assim como as gerações F₁, F₂, e retrocruzamentos (RC₁P₁ e RC₁P₂). Avaliaram-se plantas individuais após 70 dias da inoculação com o patógeno, por meio do fator de reprodução do nematoide (FR). A hipótese de herança monogênica foi rejeitada pelo teste do qui-quadrado (χ²), indicando que a resistência está sob controle de mais de um loci gênico, sendo confirmado pela análise quantitativa, que evidenciou a presença de seis genes.

Palavras-chave:
Cucumis melo; herança genética; melhoramento de plantas; fator de reprodução

INTRODUCTION:

On a national scale, the melon (Cucumis melo L.) has been of great importance as a Brazilian export. Main destinations include the Netherlands, Spain, the United Kingdom, and Italy (AGRIANUAL, 2015). In addition, it has a significant social and economic impact, since its cultivation mainly requires qualified and intensive labor.

With an increase in production, the difficulty of cultivating this vegetable has also escalated, particularly due to the infestation of production areas by numerous phytopathogens, among which root-knot nematodes (Meloidogyne ssp.) cause the most severe damage. The disease is characterized by the destruction of root cells, chlorosis, and a reduction in leaf area and fruit quality, which results in decreased crop productivity (GALATTI et al., 2013).

The most important and frequent root-knot nematodes reported in Brazilian cucurbit crops are Meloidogyne incognita (Kofoid & White, 1919) Chitwood, 1949; M. javanica (Treub) Chitwood; and M. arenaria (Neal) Chitwood (PINHEIRO & AMARO, 2010).

According to PEIL (2003), the problems of soil infestation by root-knot nematodes are increasingly difficult to solve by traditional control methods. Therefore, the use of genetic resistance is an excellent form of control because it does not add to production costs nor does it contaminate the environment.

Inheritance studies for this trait were already carried out in lettuce (Lactuca sativa) for races 1, 2, 3, and 4 of M. incognita (GOMES et al., 2000) and M. javanica (MALUF et al., 2002). These results showed that genetic control is exerted at a single gene locus and features a predominantly addictive effect, presenting relatively high broad-sense heritability.

Information regarding to the genetic control of traits linked to root-knot nematode resistance in melon is scarce in the literature. Such information is vital for decision-making about the appropriate breeding method and the size of the population to be assessed during the implementation of a breeding program aiming to obtain resistant lines.

The identification of resistance to M. incognita in the ‘Gaúcho Redondo’ cultivar, carried out by ITO et al. (2014), facilitated the study of inheritance for the trait in question. According to CRUZ & REGAZZI (1994), a genetic control study with generations F₁, F₂ and backcrosses, including parental P₁ and P₂, can lead to the quantification of the magnitude and nature of genetic variability in the segregating population and the assessment of the relative importance of genetic effects, which compose the averages of the studied populations.

Taking this into account, our study aimed to investigate the genetic control of melon resistance to M. incognita by estimating genetic parameters.

MATERIALS AND METHODS:

The experiment was carried out in the Horticultural and Medicinal Plants Sector of the Department of Crop Production at São Paulo State University-UNESP/FCAV, Jaboticabal Campus.

Cross-breeding to obtain the F₁ hybrid, segregating generation (F₂) and backcross generations (RC₁P₁ and RC₁P₂).

The susceptible strain JAB 20 (Cucumis melo var. cantalupensis Naud.) was previously crossed with the ‘Gaúcho Redondo’ cultivar (Cucumis melo var. reticulatus Naud.), which is resistant to M. incognita, in order to obtain the F₁, RC₁P₁ and RC₁P₂, and F₂ populations.

The ‘Gaúcho Redondo’ cultivar was used as a female parent and the JAB 20 strain as a male parent. This strain was developed by the melon cross-breeding program of UNESP/FCAV and has high general combining ability (GCA), especially for mass (kg fruit^-1) and fruit production (kg m^-2) (VARGAS et al., 2010).

The seeds of the aforementioned genotypes were sown in trays of expanded polystyrene, with 128 cells each, and filled with Bioplant^® substrate. Seedlings were individually transplanted to plastic pots with a capacity of 13L. Pots were previously filled with coconut husk fiber.

As soon as flowering began, artificial hybridization was carried out between progenitors, which differ in their resistance to M. incognita, and F₁ seeds were obtained. Backcross generations were obtained from crosses RC₁P₁ (F₁ x P₁) and RC₁P₂ (F₁ x P₂). In order to obtain the segregating generation (F₂), female flowers of F₁ plants were self-fertilized.

Inheritance of resistance

Once seeds from the generations involved in this study were obtained, the resistance test for M. incognita was carried out. A randomized block experimental design, with three repetitions, was adopted. Treatments were P₁ P₂, F₁, RC₁P₁, and RC₁P₂ and F₂. Experimental plots contained 10 plants for progenitors and F₁, 40 plants for backcross generations and 70 plants for F₂, for each block, respectively. Thus, the final population assessed of each generation consisted of 26 and 29 plants for progenitors P₁ and P₂, respectively, and 30, 209, 118, and 115 plants for F₁, F₂, RC₁P₁ and RC₁P₂ generations, respectively.

Several clay pots, with a capacity of 3L, were used, containing a mixture of coarse sand and soil as substrate, in a 3:1 ratio. The mixture was previously sieved and autoclaved at 120°C and 1atm, for 1 hour.

During transplantation, each plant was inoculated with 5.0mL of a suspension containing 3,000 eggs and second stage juveniles of M. incognita using an automated pipette. This was designated as the initial population (Pi). Eggs and second stage juveniles were extracted from cotton plants, and the species identifications were confirmed by examining morphological traits of the perineal pattern with an optical microscope.

In order to assess the viability and quality of the inoculum during nematode development, the same inoculum was used on tomato plants. Ten plants from a susceptible cultivar, ‘Santa Cruz Kada’, and ten plants from a resistant cultivar, ‘AP-529’, were used. The inoculum was prepared according to the technique described by HUSSEY & BARKER (1973), with the modifications introduced by BONETTI & FERRAZ (1981).

Assessment of plants inoculated with M. incognita

Seventy days after transplant and inoculation with nematodes, all inoculated plant generations were analyzed. The nematode extraction was carried out according to the technique described by HUSSEY & BARKER (1973). The final population (Pf) of each suspension was used to determine the reproduction factor (RF), in which plants with a RF<1 were considered resistant, and plants with a FR≥1 were considered susceptible to the nematode, according to OOSTENBINK (1966).

Analysis of results

The data were subjected to a quantitative and qualitative analysis. For the qualitative analysis, plants from each generation were separated into resistant (R) or susceptible (S), according to the results of the RF values. A chi-square test (χ²) was used to verify the hypothesis of dominant monogenic inheritance (3R:1S).

Quantitative genetic analyses were carried out using the software GENES, developed by the Universidade Federal de Viçosa (VFU) (CRUZ, 2013). According to CRUZ (2006), the following genetic parameters were estimated: Additive Variance ( ), Dominant Variance , Environmental Variance and Genetic Variance , Broad-Sense Heritability

and Narrow-Sense Heritability , Average Dominance Index , Gain by Selection (∆G = Ds x ) and Number of Genes .

RESULTS AND DISCUSSION:

After the inoculation with the nematode M. incognita, all parental plants P₁ were resistant, while parental plants P₂ were susceptible (Table 1). According to the results of the chi-square test with a 1% level of significance, it was possible to reject the hypothesis of monogenic inheritance, as results indicated that more genes are involved in the control of resistance.

Table 1
Reaction of melon plants inoculated with Meloidogyne incognita assessed by the reproduction factor (RF), considering the number of eggs + juveniles (J2). UNESP/FCAV, Jaboticabal-SP, 2013.

• ⁽¹⁾RF Values ≥1.0 indicate susceptibility and RF<1.0 indicate resistance to Meloidogyne incognita.
• ⁽²⁾Frequency of resistant (R) (RF<1.0) and susceptible plants (S) (FR≥1.0) in the analyzed generations.
• ^(a)Melon ‘Gaúcho Redondo’ (resistant to the nematode).
• ^(b)Melon strain JAB 20 (susceptible to the nematode).

Based on the quantitative analysis, the inheritance of the resistance trait to M. incognita may be defined as polygenic, with an estimated involvement of 6.07 genes (Table 2). Results of our study differ from those obtained by CARVALHO FILHO et al. (2008) and SOBRINHO-SOUZA et al. (2002) who examined the genetic control of resistance of lettuce ‘Salinas 88’ to M. incognita race 1 and pepper ‘Carolina Cayenne’ to M. incognita race 2, respectively. They verified the presence of only one major gene with additive effects regulating resistance, and thereby concluded that, depending on the culture studied, the genetic control of resistance to M. incognita might be regulated by several different genes.

Table 2
Estimates of the genetic parameters obtained through variance analyses for resistance to Meloidogyne incognita in the F₂ population. UNESP/FCAV, Jaboticabal-SP, 2013.

FERREIRA et al. (2010) studied the resistance of cultivars of Phaseolus vulgaris L. to M. incognita (races 1 and 3) and M. javanica. Their results showed that resistance might be controlled by different genes, depending on the species and race of Meloidogyne considered in the study, thus confirming that a different genetic control may be observed in different cultures, as reported in our study with melon ‘Gaúcho Redondo’.

Transgressive segregation was observed for the F₂ generation, both for susceptibility and resistance, with some plants showing a maximum RF of 27.87, and others a minimum RF of 0.08. This shows average RF values outside the upper and lower resistance limits of the progenitors, suggesting the occurrence of an additional gene involved in the determination of resistance or presence of epistasis. In their study of inheritance of resistance to potyvirus Pepper yellow mosaic virus (PepYMV) in tomato plants, JUHÁSZ et al. (2008) explained transgressive segregation in the F₂ generation by the presence of an additional gene involved in the determination of resistance.

In our study, broad-sense and narrow-sense heritability were 76.91% and 72.17%, respectively, showing that the resistance trait to M. incognita is determined to a greater extent by additive gene action; and therefore, it responds favorably to the selection based on individuals with lower RF values (Table 2). Similar results were obtained by CARVALHO FILHO et al. (2011), who studied population parameters and correlations among characteristics of resistance to M. incognita in lettuce. They obtained a 72% broad-sense heritability for the number of eggs, showing the possibility for success in the selection of plants resistant to M. incognita race 1. Regarding the root-knot index, CARVALHO FILHO et al. (2012) obtained a 74% broad-sense heritability in progenies of lettuce F_2:3 derived from a cross between ‘Salinas 88’ and ‘Colorado’.

The average dominance index obtained in this study was 0.36, showing an allelic interaction of incomplete dominance (Table 2). Similar results were obtained by FERREIRA et al. (2012) in a study of genetic control of resistance to M. incognita race 1 in Phaseolus vulgaris L.; incomplete dominance of the allele that controls resistance was observed.

Genotypic variance observed in the F₂ generation in this study might be attributed primarily to additive genetic effects, since the variance estimates of this effect were substantially greater than the variance estimates of dominance (Table 2). Furthermore, it was possible to observe that the genetic gain predicted by selection in the F₂ generation, based on plants resistant to M. incognita, was -63.99% (Table 2). Negative value indicated that selection occurs for plants with lower RF values.

Genetic effects observed in our study, specifically those involved in the inheritance of the resistance to nematodes, may be explained to a large extent by the additive-dominant model, represented by the sum of the effects of the mean (m), additive gene action (a), and dominance (d), since the correlation between the averages estimated by the model and the averages obtained from the generations was high (R² = 0.9745). In their study of bean plants with resistance to M. incognita race 1, FERREIRA et al. (2012) also verified that the additive-dominant model was sufficient to explain the data, since the correlation obtained was also high (R² = 0.9881).

In the additive-dominant model, the estimate of the additive genetic effect was greater than that of dominance (a = 33.96% and d = 5.29%) (Table 3). Thus, the additive genetic portion involved in the trait studied is significant; it positively responds to the phenotype-based selection of segregating generations.

Table 3
Non-orthogonal decomposition of the square sum (SS) and determination coefficient (R²) of the parameters (m, a, d) for the additive-dominant model and parameters (m, a, d, aa, ad, dd) for the complete model, based on the averages of the reproduction factor (RF) of six generations (P₁, P₂, F₁, F₂, RC₁P₁ and RC₁P₂), in populations of Cucumis melo inoculated with Meloidogyne incognita. UNESP/FCAV, Jaboticabal-SP, 2013.

• m = mean effect; a = additive effect; d = dominance deviation effect; aa = additive-additive effect; ad = additive-dominant effect; dd = dominant-dominant effect.

The complete model, represented by the sum of effects of the mean (m), additive (a), dominance (d), and their interactions (aa, ad, dd), showed that the effect of epistasis contributed 10.69% (R²) of the total variability, indicating that the epistatic effects were not important in the genetic control of the analyzed trait (Table 3).

Because the inheritance of resistance is controlled by 6.07 genes, the method of backcross generations, widely used to transfer resistance genes in plants, would not be appropriate in this particular case of resistance. The transfer of resistance genes would require the development of more complex selection strategies, such as a recurrent selection.

CONCLUSION:

An analysis of generations is effective in determining the number of genes involved in the inheritance of resistance to M. incognita. In C. melo, the genetic control of resistance to M. incognita is polygenic and ruled by six gene loci, with incomplete dominance of the resistance allele and the presence of additive genetic effects.

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• 0
  CR-2015-1147.R1

Publication Dates

• Publication in this collection
  2017

History

• Received
  11 Aug 2015
• Accepted
  26 Jan 2017
• Reviewed
  18 Feb 2017