Published online 19 March 2008
Published in Crop Sci 48:527-532 (2008)
© 2008 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
Antibiosis Resistance of QTL Introgressive Soybean Lines to Common Cutworm (Spodoptera litura Fabricius)
Kunihiko Komatsu*,
Masakazu Takahashi and
Yoshinori Nakazawa
National Agricultural Research Center for Kyushu Okinawa Region, 2421 Suya, Koushi, Kumamoto 861-1192, Japan
* Corresponding author (kkomatsu{at}affrc.go.jp).
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ABSTRACT
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To develop soybean [Glycine max (L.) Merr.] cultivars resistant to common cutworm (CCW; Spodoptera litura Fabricius), it is important to clarify the effects and genetic characteristics of quantitative trait loci (QTLs) for resistance. In a prior study, we detected two QTLs, named CCW-1 and CCW-2, for resistance to CCW in a resistant germplasm, ‘Himeshirazu’. Other researchers detected a major QTL for herbivorous insect resistance in PI 229358. Here we constructed near isogenic lines (NILs) of the QTLs via recurrent backcrossing and evaluated the effect of each QTL on antibiosis resistance to CCW. To confirm allelism between CCW-1 and the PI 229358-derived QTL, we did a crossing test. The NILs exhibited significant resistance to CCW compared with the susceptible parent, although the resistance varied between years. The effects of CCW-1 and CCW-2 were similar, and no allelic interaction between them was detected. The crossing test confirmed that CCW-1 and the PI 229358-derived QTL were identical. But the alleles could be different, because the NILs derived from Himeshirazu and PI 229358 showed different levels of resistance.
Abbreviations: CCW, common cutworm • cM, centimorgan • LG, linkage group • MAS, marker-assisted selection • NIL, near isogenic line • QTL, quantitative trait locus • SII, standardized insect-growth index • SSR, simple sequence repeat
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INTRODUCTION
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COMMON CUTWORM (CCW; Spodoptera litura Fabricius) is a serious pest of soybean [Glycine max (L.) Merr.] in southwestern Japan. Thus, soybean cultivars with resistance to CCW are required. In breeding programs, the forage cultivar Himeshirazu is used as a donor parent, as it shows effective antibiosis resistance to CCW (Komatsu et al., 2004). We genetically dissected the antibiosis resistance, and detected two quantitative trait loci (QTLs), CCW-1 and CCW-2, on linkage group (LG)-M (Komatsu et al., 2005). However, the actual effect of each QTL on the resistance was unclear, because it was estimated by algorithm. To make effective use of the QTLs in breeding programs it is necessary to verify their actual effect.
In the southern United States, damage to soybean due to herbivorous insects is also serious. Van Duyn et al. (1971) found three soybean germplasm lines with resistance to herbivorous insects and genetically analyzed them. One, named PI 229358, exhibits effective resistance and has been well analyzed. Rector et al. (2000) conducted a QTL analysis of antibiosis resistance to corn earworm (Helicoverpa zea Boddie) and detected three QTLs, on LG-G, -J, and -M. The one on LG-M plays a leading role in the control of antibiosis resistance (Narvel et al., 2001; Zhu et al., 2006). From its position and characteristics (Komatsu et al., 2005), we suspected that CCW-1 in Himeshirazu was identical to the QTL on LG-M. To utilize the PI 229358–derived QTL in breeding programs, it is necessary to confirm its practical effect on CCW growth. It is also necessary to test for allelism between CCW-1 and the PI 229358–derived QTL. Thus, we also constructed a near isogenic line (NIL) with the PI 229358–derived QTL on LG-M and evaluated its antibiosis resistance to CCW. In addition, we investigated the allelism between CCW-1 and the PI 229358–derived QTL on LG-M by a crossing test of the NILs.
The objectives of this study were (i) to confirm the effects of CCW-1, CCW-2, and the PI 229358–derived QTL on antibiosis resistance to CCW and (ii) to test the allelism of CCW-1 and the PI 229358–derived QTL on LG-M.
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MATERIALS AND METHODS
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Plant Materials
Himeshirazu, PI 229358, and ‘Fukuyutaka’ were the plant materials that formed the basis of this study. Himeshirazu is a CCW-resistant forage cultivar with two QTLs for resistance (Komatsu et al., 2005). It was developed from a progeny of a cross between ‘Misawo’ and insect resistant ‘Bansei-aoshouryu’. PI 229358 is a herbivorous insect–resistant Japanese germplasm that has been well analyzed genetically (Rector et al., 1998, 1999, 2000; Narvel et al., 2001; Zhu et al., 2006). Its antibiosis resistance to CCW has been confirmed (Komatsu et al., 2004). It is difficult to verify directly if PI 229358 is an ancestor of Himeshirazu or not, because Bansei-aoshouryu, the insect resistance donor parent of Himesirazu, has been lost from Gene-bank of Japan. But, two simple sequence repeat (SSR) markers (Sat_258 and Satt175) closely located to the QTL for insect resistance show polymorphism in Himeshirazu and PI 229358 (data not shown). Thus we think that PI 229358 was not an ancestor to Himeshirazu. Fukuyutaka is a leading cultivar in southwestern Japan. It is susceptible to CCW and was used as a parent of the segregating population used in the QTL analysis of CCW resistance (Komatsu et al., 2005). Three NILs (NIL-C1, NIL-C2, and NIL-C1+C2) were developed via recurrent backcross using Fukuyutaka as the recurrent parent. NIL-C1 was homozygous for the Himeshirazu allele at CCW-1, and homozygous for the Fukuyutaka allele at CCW-2. NIL-C2 was homozygous for the Fukuyutaka allele at CCW-1, and homozygous for the Himeshirazu allele at CCW-2. NIL-C1+C2 was homozygous for the Himeshirazu allele at both loci. All selection during the development of NILs was assisted by SSR markers. NIL-PI, for the herbivore insect resistance QTL on LG-M from PI 229358, was also developed via recurrent backcross. In addition, three lines heterozygous at the CCW-1 locus or the PI 229358 QTL were bred from Fukuyutaka and the above NILs: Fukuyutaka x the NIL for CCW-1 (Fuku/NIL-C1), Fukuyutaka x the NIL derived from PI 229358 (Fuku/NIL-PI), and the NIL for CCW-1 x the NIL derived from PI 229358 (NIL-C1/NIL-PI). The genetic relationships of all plant materials are shown in Fig. 1
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Figure 1. Development of near-isogenic lines (NILs) with quantitative trait loci (QTLs) for antibiosis resistance; ‘Fuku’ indicates recurrent parent, ‘Fukuyutaka’. Boxed materials were evaluated the antibiosis resistance to common cutworm in this study. NIL-C1+C2 is homozygous for the resistant Himeshirazu allele at CCW-1 and CCW-2 loci. NIL-C1 is homozygous for the Himeshirazu allele at CCW-1. NIL-C2 is homozygous for the Himeshirazu allele at CCW-2. NIL-PI is homozygous for the PI 229358 allele at insect resistance QTL on linkage group M. Fuku/NIL-C1 and Fuku/NIL-PI are F1 hybrids of Fukuyutaka and NIL-C1 or NIL-PI. NIL-C1/NIL-PI is the F1 of NIL-C1 and NIL-PI.
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In addition to the QTL on LG-M, another QTL on LG-G for antibiosis resistance to corn earworm has been reported in PI 229358 (Rector et al., 2000). It has epistatic interaction with QTL on LG-M (Zhu et al., 2006). Thus, the genotype of the QTL on LG-G was estimated in all NILs with flanking SSR markers Satt472, Satt191, and Sat_117. In all NILs, the region was estimated to be fixed for Fukuyutaka allele (data not shown). Therefore we feel that epistatic interactions were not a factor in this study.
Marker-Assisted Selection and the Drawing of a Graphical Genotype of LG-M
Marker-assisted selection (MAS) and the drawing of a graphical genotype of LG-M were performed with SSR markers. Except for VSP_A and T-5-23, all SSR loci used in this study were published by Cregan et al. (1999) or in SoyBase (http://www.soybase.org/). VSP_A and T-5-23 were developed by K. Harada at Chiba University (personal communication, February 2002). For MAS at the CCW-1 locus, we used Satt220, Sat_258, and Satt175. For CCW-2, we used Satt567 and Satt463. For the QTL from PI 229358, we used Sat_258 and Sat_288. These SSR pairs flanked each targeted QTL (Fig. 2
). Every MAS in this study was performed at the seed stage, as described in the next section. Only seeds whose SSR genotypes were either heterozygous or homozygous for the donor parent allele were sown. The graphical genotypes of all plant materials are shown in Fig. 2. To draw the graphical genotype of NILs derived from Himeshirazu, we used SSR loci on LG-M that appeared in a prior genetic study of Himeshirazu (Komatsu et al., 2005). In addition to those, we used Sat_258 and Sct_147 to make a more detailed drawing. The positions of Sat_258 and Sct_147 were confirmed, using DNA samples of an F2 population used in previous studies (Komatsu et al., 2004, 2005). To draw the graphical genotype of NILs related to PI 229358, we used all the SSR loci on LG-M that were polymorphic between Fukuyutaka and PI 229358. The linkage relationship was based on the report of Cregan et al. (1999).
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Figure 2. Graphical genotypes of linkage group M and standardized insect-growth index (SII) of near isogenic lines (NILs). ‘Fuku’ indicates recurrent parent, ‘Fukuyutaka’. The dark gray and light gray bars indicate regions from ‘Himeshirazu’ and ‘PI 229358’, respectively. The white bars indicate Fukuyutaka. The positional relationships of simple sequence repeat (SSR) markers are based on the linkage map of Komatsu et al. (2005) for Himeshirazu-derived NILs and on that of Cregan et al. (1999) for PI 229358–derived NILs. Within a column, SIIs followed by different letters are significantly different as determined by the Tukey–Kramer multiple comparison test (P < 0.05).
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DNA Extraction and Genotyping of SSR Loci
DNA for MAS and the drawing of graphical genotypes was extracted from the seeds. We scraped off about 50 mg of cotyledon with a sharp scalpel, carefully retaining the hypocotyl to maintain germinability. We also removed the seed coat, as it originates from the maternal parent, and thus its genotype could differ from that of the embryo. The cotyledon sample was crushed to a powder with a thin steel stick in a 1.5-mL collection tube. DNA was extracted with a MagExtractor Plant Genome Kit (Toyobo Co., Osaka, Japan) or a DNeasy Plant Mini Kit (Qiagen Co., Hilden, Germany). The polymerase chain reaction conditions and electrophoresis methods were as previously reported (Komatsu et al., 2004, 2005).
Evaluation of Antibiosis Resistance to Common Cutworm
We evaluated the antibiotic effect of each line on CCW with the standardized insect-growth index (SII). The SII is a measure of growth rate, calculated as (pupal weight)/(duration of sixth instar). The value increases as the antibiosis of the soybean line used as diet decreases. The procedure for the bioassay has been reported in detail (Komatsu et al., 2004). The conditions and methods were the same as reported, except that the origin of the larvae used in the bioassay and the range of the sixth instar larval weights differed. Previously, the larvae originated from a population maintained at the laboratory of the National Agricultural Research Center for the Kyushu Okinawa Region, and the weight ranged between 180 and 300 mg. Here, the larvae were obtained from Sumika Technoservice Co. (Hyogo, Japan), and the weight ranged between 320 and 380 mg. We chose the CCW population from Sumika Technoservice Co. because of the low variance of larval weight. For evaluation of antibiosis, we used 48 CCW larvae per line (six CCW per plant, eight plants per line) in both 2005 and 2006. In 2005, seeds were sown on 30 May, and the bioassay started on 2 August. In 2006, seeds were sown on 4 June and the bioassay started on 14 August. We evaluated the antibiosis of Fukuyutaka, Himeshirazu, NIL-C1, and NIL-C1+C2 in both years with a two-way analysis of variance (ANOVA) of SII, in which the soybean line and the year were set as dimensions. As Fukuyutaka, NIL-C1, NIL-C2, and NIL-C1+C2 are NILs at the CCW-1 and CCW-2 loci, all possible homozygous genotypes at the two loci are completed. Thus, it was possible to investigate the genetic interaction between CCW-1 and CCW-2. Zhu et al. (2006) reported the epistatic interaction of QTLs for insect resistance in soybean, thus the interaction between CCW-1 and CCW-2 should be investigated. We performed a two-way ANOVA on SII in 2006, in which the genotypes, CCW-1 and CCW-2, were set as the main effect.
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RESULTS
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To confirm the actual effect of the insect resistance QTLs, we developed NILs by marker-assisted recurrent backcross. Their antibiosis resistance to CCW was evaluated using SII values. The results are shown in Fig. 2. The statistical significance of the difference in SIIs was determined using the Tukey–Kramer multiple comparison test at the 5% level.
In 2005, we evaluated the antibiosis resistance to CCW of Himeshirazu and PI 229358 as resistant germplasms, Fukuyutaka as a susceptible cultivar, Himeshirazu-derived NIL-C1 and NIL-C1+C2, and PI 229358–derived NIL-PI. In addition to the genetically fixed lines, we investigated three heterozygous F1 lines obtained from Fukuyutaka x NIL-C1, Fukuyutaka x NIL-PI, and NIL-C1 x NIL-PI. The two resistant germplasms showed lower SII values than all other lines, exhibiting the highest antibiosis resistance to CCW. Fukuyutaka showed the lowest antibiosis resistance. The SIIs of NIL-C1, NIL-C1+C2, and NIL-PI were all significantly lower than that of Fukuyutaka. NIL-C1/NIL-PI also exhibited a lower SII than Fukuyutaka, but the SIIs of Fuku/NIL-C1 and Fuku/NIL-PI were not significantly different from that of Fukuyutaka.
In 2006, we investigated Himeshirazu, Fukuyutaka, NIL-C1, NIL-C1+C2, and NIL-C2. The NIL-C2 was derived from Himeshirazu and analyzed for the first time for antibiosis resistance. As in 2005 the SII value of Himeshirazu was the lowest and that of Fukuyutaka was the highest in all the plant materials. NIL-C1+C2 had a statistically lower SII compared with the other two NILs. The SII values of NIL-C1 and NIL-C2 were significantly higher compared with that of Himeshirazu and NIL-C1+C2, but significantly lower than that of Fukuyutaka.
The results of the two-way ANOVA in which soybean line and year were set as main effects are shown in Table 1
. Soybean line, year, and the interaction of soybean line and year were significant (P < 0.01) as estimated by an F-test. The results of the two-way ANOVA in which genotype was designated as the main effect are shown in Table 2
. The genotypes at both loci were significant (P < 0.01), but the interaction between CCW-1 and CCW-2 was not significant.
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Table 1. Two-way analysis of variance for the standardized insect-growth index of four genotypes evaluated in 2005 and 2006.
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Table 2. Two-way analysis of variance of the effect of CCW-1 and CCW-2 loci on the standardized insect-growth index.
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DISCUSSION
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Two-way ANOVA of soybean line and examination year revealed significant differences between lines, years, and their interaction. It is reasonable that significant differences should exist in the SII of the soybean lines because the lines include resistant Himeshirazu and susceptible Fukuyutaka. We reported this difference previously (Komatsu et al., 2004). On the other hand, the significant effects of year and line x year interaction on variance in SII indicate that it is not reasonable to compare the SIIs between 2005 and 2006 directly. It is difficult to be sure of the cause of this difference, but we think that environmental differences between years could affect resistance. Because the plants were planted in the field (albeit in a vinyl house), it was almost impossible to regulate the biotic and climatic effects on them. The absolute values of SII were different between years, yet the relative relationships of SIIs of the plant materials were similar in both years. Thus, we believe it is possible to compare the effects of QTLs if the comparison is limited to the results within each year.
The SIIs of NIL-C1 and NIL-PI in 2005 confirmed the significant effects of CCW-1 and the QTL on LG-M of PI 229358. This fact is evidence for two important forward steps in the breeding of CCW-resistant soybean. One is verification of the effect. We previously reported the effect of CCW-1 on CCW (Komatsu et al., 2005), but this result was based on QTL analysis and was thus statistically presumed. In contrast, there has been no report of the effect of the PI 229358–derived QTL on CCW. The other step is verification of MAS using the QTL. The NILs were developed via marker-assisted recurrent backcross with no confirmation of the antibiosis in each generation. A similar strategy for selection (for example, two crosses a year with no confirmation of antibiosis) would be useful in a breeding program.
The result in 2005 confirmed the allelism between CCW-1 and the QTL of PI 229358. In our prior report, we estimated the resistant allele of CCW-1 to be recessive (Komatsu et al., 2005). Rector et al. (2000) estimated that the resistant allele of QTL of PI 229358 is also recessive. In this study, we confirmed those estimations because Fuku/NIL-C1 and Fuku/NIL-PI did not have significant resistance compared with Fukuyutaka. On the other hand, NIL-C1/NIL-PI had significantly higher resistance than Fukuyutaka. If CCW-1 and the QTL of PI 229358 were different loci, NIL-C1/NIL-PI should exhibit low resistance because both resistant alleles are recessive. However, NIL-C1/NIL-PI exhibited high resistance. Thus, it is certain that CCW-1 and the QTL of PI 229358 are identical. An interesting fact about the locus is that the SIIs of NIL-C1 and NIL-PI were significantly different. The QTL on LG-G interacting with PI 229358–derived QTL on LG-M was fixed for the Fukuyutaka allele in all NIL materials, thus the genetic interaction between them is probably not the cause. It is difficult to identify the cause of the difference between NIL-C1 and NIL-PI in this study, but it is possible that the alleles are different and have different effects, although they reside at the same locus. If this is so, it is also interesting to compare the effect of the resistant allele of PI171451 with the Himeshirazu and PI 229358 alleles, because a resistant allele was also detected in PI171451 (Rector et al., 2000) but investigation of the allele has not been done.
In 2006, we confirmed the effect of CCW-2 with NIL-C2. This is the first confirmation of the actual effect of the locus. In addition, the SII of NIL-C2 was close to that of NIL-C1, and thus the effect of CCW-2 is similar to that of CCW-1. So CCW-2 could also play an important role in breeding programs for CCW resistance. The two-way ANOVA for the CCW-1 and CCW-2 loci indicated that each of the loci had an effect on antibiosis resistance, but there was no significant interaction between them. This suggests that there is no epistasis between CCW-1 and CCW-2. In contrast, Zhu et al. (2006) reported epistatic interaction between insect resistance QTLs on LG-M and LG-G from PI 229358. These genetic relationships among the herbivorous insect resistance QTLs are very interesting. The interaction between CCW-2 and the PI 229358–derived QTL on LG-G should be investigated. Clarification of the relationships among the QTLs would contribute to our understanding of the mechanism of insect resistance in soybean. On the other hand, the low stability of the SII value of NIL-C1+C2, which was fixed for resistant alleles at CCW-1 and CCW-2, should be considered. The SIIs of NIL-C1+C2 differed significantly between 2005 and 2006, although the SIIs of NIL-C1 were almost the same. The reason for this phenomenon is not clear. One possibility is an influence of plant development stage on antibiosis resistance. A QTL for maturity has been detected near the CCW-2 (Orf et al., 1999), and plant development stage could affect plant antibiosis resistance to insects (Nault et al., 1992). In addition, the period of the bioassay in our study differed slightly in 2005 and 2006. The difference in development stage of the NILs at the bioassay period could be a cause of instability of NIL-C1+C2. Nevertheless, no QTL for days to flowering has been detected on LG-M in the F2 segregating population of Fukuyutaka and Himeshirazu (Komatsu et al., 2007). In addition, NIL-C1+C2 flowered and matured at the same time as Fukuyutaka in 2005 and 2006, and NIL-C2 also flowered and matured the same as Fukuyutaka in 2006 (data not shown). Thus, we believe that the Himeshirazu-derived region of NIL-C1+C2 does not affect the maturity, and plant development stage might not be a cause of the instability in the SII of NIL-C1+C2.
The PI 229358–derived QTL on LG-M, which our results show is identical to CCW-1, has been well analyzed. Zhu et al. (2006) estimated that it is located in a 0.52-cM space between SSR loci Sat_258 and Satt702. The precision of the SSR markers as instruments for selection in a breeding program should be adequate. Even positional cloning of the gene might be possible with the SSR loci. In addition, the genetic interaction between the QTLs on LG-M and LG-G, both from PI 229358, was revealed (Zhu et al., 2006). The QTL on LG-G displays the antibiosis effect to corn earworm under the resistant allele of the QTL on LG-M. Thus, a line fixed for resistance alleles at both loci exhibits higher antibiosis resistance than a line with only QTL-M–derived resistance. However, the practical effect of the loci in the field is still unclear. If the resistance conferred by these genes is not sufficient, another resistance gene would need to be introduced. Even if the gene on LG-M is cloned and its role in resistance is determined, other genes for resistance should still be investigated to explain the whole system of herbivorous insect resistance in soybean. We confirmed the effect of CCW-2 in this study for the first time, and found the possibility of another resistant allele different from the PI 229358 allele at CCW-1. Further progress in investigation of the genes of Himeshirazu would contribute not only to the development of insect-resistant soybean cultivars, but also to our understanding of the mechanisms of insect resistance in soybean.
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ACKNOWLEDGMENTS
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We thank Prof. Kyuuya Harada at Chiba University for the SSR information. This work was supported by a grant (Green Technology Project DM-1204) from the Ministry of Agriculture, Forestry and Fisheries of Japan.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication December 7, 2006.
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ABSTRACT
INTRODUCTION
