Indirect response to selection for improving resistance to the Mediterranean corn borer ( Sesamia nonagrioides Lef) in maize

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Mediterranean corn borer (MCB) (Sesamia nonagrioides Lef) and European corn borer (ECB) (Ostrinia nubilalis Hbn) are the most important biotic stresses of maize in Europe. The first selection program to improve stalk resistance to MCB was carried out
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   1 Indirect response to selection for improving resistance to the Mediterranean corn 1 borer ( Sesamia nonagrioides  Lef) in maize 2 3 Sandoya G, Butrón A, Santiago R, Alvarez A & Malvar RA 4 5 Sandoya G, Butrón A, Santiago R & Malvar RA 6 Misión Biológica de Galicia, CISIC, Apartado 28, 36080 Pontevedra, Spain. 7 gsandoya@mbg.cesga.es; abutron@mbg.cesga.es; rsantiago@mbg.cesga.es ; 8 rmalvar@mbg.cesga.es 9 10 Alvarez A 11 Estación Experimental de Aula Dei, CSIC, Apartado 202, 50080 Zaragoza, Spain. 12 aalvarez@eead.csic.es 13 14 Corresponding author:  gsandoya@mbg.cesga.es 15 Misión Biológica de Galicia, CISIC, Apartado 28, 36080 Pontevedra, Spain. 16 17 Short Title:  Indirect response to MCB ( Sesamia nonagrioides  Lef) selection 18 19   2 Abstract Mediterranean corn borer (MCB) ( Sesamia nonagrioides  Lef) and European 1 corn borer (ECB) ( Ostrinia nubilalis  Hbn) are the most important biotic stresses of 2 maize in Europe. The first selection program to improve stalk resistance to MCB was 3 carried out in the maize population EPS12. It has shown that selection was effective to 4 improve stalk resistance to MCB and European corn borer (ECB), while yield was not 5 significantly diminished. The objective of this research was to determine if correlated 6 changes in EPS12 occurred due to selection for resistance to MCB. Cycles of selection 7  per se  and testcrosses to three testers were evaluated under MCB and ECB artificial 8 infestation at two different Spanish locations during two years. Selection has 9 significantly reduced cob damage, days to silking, plant and ear height, and 100-kernel 10 weight; meanwhile early vigor was increased. These changes could rather be a 11 consequence of unconscious selection and/or the genetic correlation of these traits with 12 resistance than a consequence of genetic drift. 13 14 Key words:  insect resistance, intrapopulation Mediterranean Corn Borer, maize, 15 recurrent selection, Sesamia nonagrioides  Lef 16   3 Introduction 1 2 European corn borer (ECB) ( Ostrinia nubilalis  Hbn) is one of the most destructive 3 maize (  Zea mays  L.) pests in North America and northern and central Europe while 4 Mediterranean corn borer (MCB) ( Sesamia nonagrioides  Lef), also known as pink stem 5  borer, is the most destructive pest in the Mediterranean area, causing important stalk 6 damage and decreases on maize yield. Several selection programs have been carried out 7 to improve maize resistance to ECB (Russel et al., 1979; Klenke et al., 1986; Nyhus et 8 al., 1989; Anglade et al., 1996). There is only one report about selection done for 9 improving resistance to MCB, three cycles of S 1  intrapopulational recurrent selection 10 have improved resistance to MCB while yield was maintained in EPS12 synthetic 11 (Sandoya et al., 2008). 12 Selection could have positive or negative effects in other resistance-related and 13 agronomic traits. Previous research indicated that recurrent selection for resistance to 14 ECB resulted in decreased yielding ability (Russell et al., 1979; Klenke et al., 1986) and 15 changes in important agronomic traits (Russell et al., 1979). In selection for resistance 16 to first generation ECB, Russell et al. (1979) reported that favorable changes in plant 17 height, ear height, and grain yield were associated with selection for resistance to insect 18 attack. Klenke et al. (1986) found that four selection cycles reduced the damage by both 19 the first and second generation of ECB attack but decreased grain yield, suggesting that 20 yield should be included in the selection criteria in a selection program. There are no 21 current studies regarding changes due to selection for resistance to MCB in agronomic 22 and resistance-related traits. 23 In general, these changes occur because the alleles involved in pest resistance 24 could have pleiotropic effects or could be in linkage disequilibrium with alleles 25   4controlling other quantitative traits of agronomic importance. Moreover, unfavorable 1 changes in agronomic traits in a recurrent selection program may be the result of 2 sampling variation for alleles affecting nonselected traits (genetic drift) if few 3 individuals are selected to form the parents of subsequent cycles (Nyhus et al., 1989). 4 Maize resistance to corn borers is usually measured by reduced stalk tunneling, 5 while ear resistance is rarely considered. Damaged ears could increase the secondary 6 infection by microorganisms and accumulation of mycotoxin (Muñoz et al., 1990; 7 Butrón et al., 2006b). Therefore the indirect response in ear resistance should be 8 considered when stalk resistance is improved. The objective of this research was to 9 determine correlated changes in agronomic and ear-resistance traits due to selection for 10 resistance against MCB attack in the maize synthetic population EPS12.11   5 Materials and Methods  1 2 Selection program of EPS12 3 The S 1  recurrent intrapopulational recurrent selection program for improving 4 Mediterranean corn borer (MCB) resistance in the maize synthetic population EPS12 5 started after obtaining three cycles of recurrent selection for yield in EPS7 (Vales et 6 al., 2001). EPS7 was srcinally made using four landraces from the Ebro Valley and 7 eastern Spain (Ordás, 1991). 8 The selection program used to improve resistance of EPS12 against Mediterranean 9 corn borer (MCB) attack started in 1993 when approximately 150 S 1 families were 10 derived from the EPS12 maize synthetic. In 1994, 100 S 1  families were evaluated in 11 field conditions under artificial infestation with MCB eggs; the families with the 12 shortest tunnel made by MCB and the yield up to the average were selected. In 1995 13 selected families were recombined and the first cycle of recurrent selection EPS12(S)C1 14 was established in 1996. In a similar manner, EPS12(S)C2 and EPS12(S)C3 were 15 obtained in 1999 and 2002, respectively. Unfortunately, EPS12(S)C1 seeds were 16 accidentally mixed with seeds from another maize synthetic, and they could not be 17 included in the present study. Nevertheless, the selection process was not affected 18  because S 1  families were obtained from EPS12(S)C1-Syn1 before recombination to 19 obtain EPS12(S)C1. More details about this selection program can be found in details in 20 Butrón et al. (2005) and Sandoya et al. (2008). 21 In 2002, seeds from the synthetics EPS12(S)C0, EPS12(S)C2 and EPS12(S)C3 were 22 multiplied and testcrossed to inbred lines A639, B93 and EP42. The inbred lines A639 23 and B93 were reported as resistant to MCB, and EP42 as susceptible (Butrón et al., 24 1999a; 2006a). 25
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