Selection of popcorn genotypes tolerant to Spodoptera frugiperda and key traits related to the identification of tolerance

The Spodoptera frugiperda, is one of the most deleterious pests of popcorn and the identification of tolerant genotypes is determinant in breeding programs. The objective of this study was to select popcorn genotypes tolerant to S. frugiperda and the key traits related to the identification of tolerance. The popcorn varieties UEM J1, Composto Márcia, Arachida, Composto Gaúcho and Zapalote Chico (resistant check) were evaluated in a completely randomized design with 100 replications. The experimental unit consisted of one Petri dish, containing plant material and a larva. The following traits were evaluated: larval stage duration (LSt), food intake weight (IW), final larva weight (FW), mean larva weight (MW), feces (F), assimilated (A) and metabolized food weight (M), relative consumption rate (RCR), relative metabolic rate (RMR), relative growth rate (RGR), conversion efficiency of ingested food (CEI), apparent digestibility (AD), conversion efficiency of digested food (CED) and leaf area consumed (LAC). The diagnosis of multicollinearity, analysis of canonical variables, genetic divergence, hierarchical clustering, factor analysis and canonical correspondence analysis were carried out to perform multivariate analysis. After the multicollinearity test, the traits FW, IW, RCR, AD and LAC were maintained for further analysis. Variety Arachida was considered tolerant to S. frugiperda and can be used in the future as a source of favorable alleles to breed tolerant popcorn hybrids. The traits relative consumption rate, apparent digestibility and leaf area consumed were considered key traits in the identification of tolerance against S. frugiperda in popcorn genotypes.

The plants were grown in a greenhouse with automatic irrigation. Crop management 93 was carried out in accordance with the recommendations for corn culture [16]. The seeds were 94 separated and three sown in each pot, which contained soil and substrate (3:1). After sowing, 95 the pots were irrigated daily and side-dressed with urea (45% N). The other cultural treatments 96 were applied as required for full crop development, without using any other chemical product, 97 so as not to affect larva growth.

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The corn leaves used to feed the larvas were collected when the plants were in the 99 eight-leaf (V8) stage, so that all genotypes were evaluated when the plants were in the same 100 developmental stage [6,7,17].

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The insects required to initiate the trial were hatched from S. frugiperda eggs donated 102 by EMBRAPA Soybean, in Londrina, Paraná, and from eggs collected in corn fields on the 103 Experimental Farm of Iguatemi -Maringá. The larvas hatched from these eggs were fed an 104 artificial diet and three generations were reared for our use in the trial.

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The laboratory trial was carried out in an air-conditioned chamber at 25°C±1, air 106 humidity of 70% ± 10 and a 12h photoperiod. Each experimental unit consisted of a sterile 107 acrylic Petri dish (diameter 9.0cm, height 1.5cm), lined with filter paper moistened with 108 distilled water to maintain the leaf turgor, containing only one larva per dish, to avoid insect 109 cannibalism. Each treatment consisted of three Petri dishes with moist filter paper and plant 110 material, to calculate the water loss.

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The trial was conducted in a completely randomized design, with five treatments and 112 100 replications.     (Table 3).

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In this study, the high commonality (from 0.7012 -A to 0.9912 -RCR), indicated traits 279 with a high relation to the determination of tolerance, confirming the thesis that the selected 280 traits can be considered key traits (Table 4). In the factor analysis, the first factor was 281 determinant for traits IW, FW, and RCR and the second for AD and LAC (Table 4). In this 282 study, estimates above 0.90 were considered for one factor and low estimates for the other factor 283 (Figure 3), which shows a high representativeness of the factor for the respective traits [25]. 295 assimilated food, M: metabolized food, RCR: relative consumption rate, RMR: relative metabolic rate, RGR: 296 relative growth rate, CEI: conversion efficiency of food intake, AD: apparent digestibility, CED: conversion 297 efficiency of the digested food and LAC: leaf area consumed. 298 299 Figure 3. Biplot of factor analysis for traits related to tolerance to Spodoptera frugiperda in the 300 composites UEM J1, Zapalote Chico, Márcia, Arachida and Gaúcho. LSt: larval stage duration, 301 IW: food intake weight, FW: final larva weight, MW: mean larva weight, F: feces, A: 302 assimilated food, M: metabolized food, RCR: relative consumption rate, RMR: relative 303 metabolic rate, RGR: relative growth rate, CEI: conversion efficiency of food intake, AD: 304 apparent digestibility, CED: conversion efficiency of the digested food and LAC: leaf area 305 consumed. 306

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Canonical correspondence analysis is an exploratory technique to simplify the 308 structure of multivariate data variability, in which the traits are arranged in contingency tables, 309 taking correspondence measures between rows and columns of the data matrix into account.
310 According to [38], correspondence analysis is a method to determine an association system 311 between the elements of two or more sets, to explain the association structure of the factors in 312 question. Thus, graphs were constructed with the principal components of the rows and 313 columns, allowing the visualization of the relationship between the sets, where the proximity 314 of the points referring to the row and the column indicates an association and distance indicates 14 316 is that relationships can be detected by this technique that would not have been perceived if the 317 analysis were based on trait pairs. In addition, it is highly flexible in the data traits, since no 318 theoretical model of probability distribution must be adopted. A rectangular matrix containing 319 non-negative data is sufficient, which in the field of breeding, makes it possible to masterfully 320 relate the effects of different traits on specific genotypes.

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In general, the selected traits were efficient in discriminating the genotypes regarding 339 tolerance and susceptibility to S. frugiperda by the applied analyses. The identification of key 340 traits in the description of tolerant genotypes will, in future studies, allow greater emphasis on 341 specific traits and consequently a more effective selection regarding tolerance in popcorn 342 genotypes.