Ó 2003 The American Genetic Association
Journal of Heredity 2003:94(6):523–527 DOI: 10.1093/jhered/esg101
Inheritance of Resistance to Clopyralid and Picloram in Yellow Starthistle (Centaurea solstitialis L.) Is Controlled by a Single Nuclear Recessive Gene R. P. SABBA, I. M. RAY, N. LOWNDS,
AND
T. M. STERLING
From the Department of Entomology, Plant Pathology and Weed Science (Sabba and Sterling) and the Department of Agronomy and Horticulture (Lownds and Ray), New Mexico State University, Las Cruces, NM 88003. R. P. Sabba is currently at the Department of Horticulture, University of Wisconsin, Madison, WI 53706. N. Lownds is currently at the Department of Horticulture, Michigan State University, East Lansing, MI 48824. Address correspondence to T. M. Sterling at the address above, or e-mail:
[email protected].
The noxious weed yellow starthistle (Centaurea solstitialis L.) can be controlled effectively at the seedling stage with foliar application of the auxinic herbicides picloram or clopyralid. Although resistance to these herbicides is rare, a yellow starthistle biotype resistant to picloram and cross-resistant to clopyralid was observed in 1989 near Dayton, WA, in a pasture that had been subjected to intensive picloram selective pressure. Our objective was to determine the mode of inheritance for this resistance trait. Transmission of the resistant phenotype was monitored in reciprocal F1 crosses between susceptible (SCI) and resistant (RDW) plants, their testcross and pseudo-F2 progeny. Progeny from all crosses, as well as RDW and SCI seedlings of original populations, were sprayed with picloram or clopyralid to distinguish between susceptible and resistant individuals. All F1 progeny were susceptible to both herbicides, indicating that the resistance trait was of nuclear origin and recessive in nature. Segregation of the resistant phenotype among pseudo-F2 and testcross progeny of F1 genotypes demonstrated monofactorial inheritance (P . .25) for resistance to both herbicides. The conclusion that resistance is conferred by a single recessive allele is consistent with the observation that no other picloramresistant yellow starthistle populations have been identified in the area since picloram selection pressure was abated.
Yellow starthistle (Centaurea solstitialis L.) is a highly competitive diploid winter annual of Eurasian origin that is advancing steadily on western rangelands of the United States. This noxious weed can be controlled effectively at the seedling stage by foliar application of the pyridinecarboxylic acid herbicides picloram or clopyralid. These compounds are
structurally similar to the plant hormone indole-3-acetic acid (IAA) and are believed to operate by similar mechanisms in susceptible plants. Symptoms of pyridinecarboxylic acid herbicide application are similar to those of other auxinic herbicides and include induction of ethylene synthesis, epinastic bending of leaves and stems, chlorosis, and eventually necrosis and death (Sterling and Hall 1997). Despite more than 50 years of auxinic herbicide use, reports of resistance to these compounds have been relatively rare. However, in field situations where auxinic herbicides have been used repeatedly, instances of herbicide resistance have been documented (Barnwell et al. 1989; Bourdot et al. 1989; Hall and Romano 1995). In 1988, a yellow starthistle population in a pasture near Dayton WA, which had a history of intensive picloram application, was reported to be resistant to picloram (Callihan et al. 1990). This observation was confirmed in 1990, coupled with the finding that the resistant plants also exhibited superior resistance to clopyralid (Fuerst et al. 1996). These reports are of major concern, as picloram and clopyralid are two of the most commonly used herbicides for yellow starthistle management in the Pacific Northwest (Callihan et al. 1989) and California (DiTomaso et al. 1999), respectively. Our research objective was to determine the mode of inheritance of auxinic herbicide resistance in yellow starthistle by crossing picloram-susceptible (SCI) plants with picloram-resistant (RDW) plants and screening F1, pseudoF2, and testcross progeny with picloram or clopyralid.
Methods Plant Material Susceptible (SCI) seeds of yellow starthistle used in this study were collected from wild-type plants growing in Central
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Grade, ID. Resistant (RDW) seeds were derived from openpollinated progeny of a single plant resistant to 0.56 kg a.e./ ha picloram from Dayton, WA. Progeny of this resistant plant were sprayed with 0.07–0.14 kg a.e./ha picloram in the greenhouse and survivors were cross-pollinated to produce RDW seed (Fuerst EP, personal communication). Growth Conditions for Parent Plants SCI and RDW seeds were germinated in petri dishes and the resulting seedlings were transplanted into 4 cm diameter pots containing potting soil and grown under greenhouse conditions, where they were fertilized weekly with Technigro 20-18-18 fertilizer (Fisons Horticulture Inc., Warwick, NY) until senescence. At the five- to seven-leaf stage (approximately 1 month after planting), 40 RDW seedlings were sprayed with 0.28 kg a.e./ha picloram (Tordon 22K, recommended rate) using a pressurized CO2 backpack sprayer to ensure that they were resistant. Fifteen of these plants (RDW1–RDW15) and 15 SCI plants (SCI1–SCI15) were chosen randomly and then transplanted into 20 cm diameter pots and allowed to bolt under natural short-day conditions in the greenhouse. Bolting plants were transplanted into 30.5 cm diameter pots and used as parents to produce F1 progeny. Generation of F1, Pseudo-F2, and Test-Cross Populations At the receptive flower stage, selected plants were paired to produce the desired crosses. Flowers were cross-pollinated by first collecting pollen on filter paper from a male donor plant. Cotton swabs were used to apply the pollen to receptive stigmas on a mother plant. Pollinated flowers were then covered with cheesecloth to prevent contamination and to facilitate seed harvest. Selfing rates were determined from flowers that had been used as pollen donors only. Each of the 15 RDW parents was randomly paired with one of the 15 SCI parents and each pair of plants was reciprocally crossed. The fertility of the crosses was very erratic and several crosses essentially failed. Five of the 15 parental single crosses produced sufficient reciprocal F1 seed for evaluation. Subsequently, however, only 2 of the 15 pairs (i.e., SCI4 3 RDW4 and SCI7 3 RDW7) produced sufficient reciprocal F1, pseudo-F2, and testcross seed for the genetic analyses described in this article. The F1 progeny from susceptible mothers (SCI4 and SCI7) are referred to as ‘‘Sseries’’ and are designated S4R4F1 and S7R7F1, respectively. The reciprocal F1 progeny from resistant mothers (RDW4 and RDW7) are referred to as ‘‘R-series’’ and are designated R4S4F1 and R7S7F1, respectively. Traditional F2 populations could not be generated by selfing F1 genotypes because yellow starthistle has a low selfing rate (Harrod and Taylor 1995; Maddox et al. 1996; Sun and Ritland 1998). Therefore a single genotype from each of the four F1 populations was subsequently paired and intercrossed (i.e., S4R4F1 3 S7R7F1 and R4S4F1 3 R7S7F1) to generate two (reciprocal) pseudo-F2 populations. Single genotypes from each of the four F1 populations were also testcrossed to susceptible (SCI) and resistant (RDW) testers.
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In addition, the RDW and SCI testers were crossed to each other to characterize their genotypes. Herbicide Screening Seedlings to be screened for herbicide resistance were sprayed at 5 weeks of age with either picloram (Tordon 22K, Dow AgroSciences, Midland, MI) or clopyralid (Reclaim, Dow AgroSciences, Midland, MI) using a pressurized CO2 backpack sprayer at the recommended rate of 0.28 kg a.e./ha. An equal number of control plants were sprayed with water. Twenty progeny from reciprocal F1 crosses and 20 SCI seedlings (control) were screened for resistance to clopyralid, and 13 to 24 each of reciprocal F1 progeny and SCI seedlings were screened for resistance to picloram. Because reproductive fertility was erratic among parents, varying numbers (1– 219) of pseudo-F2 and testcross progeny were screened with picloram or clopyralid to determine segregation ratios for the number of resistant and susceptible progeny. Data Collection and Statistical Analysis Plants were monitored at weekly intervals after herbicide treatment for survival or death. Observed segregation ratios of resistant (R) and susceptible (S) progeny for F1, pseudoF2, and testcross generations were tested for goodness-of-fit to monogenic and digenic models, assuming disomic inheritance. Digenic models involving duplicate dominant epistasis and duplicate recessive epistasis were evaluated because only parental phenotypes were observed in the pseudo-F2 populations. Data were interpreted using chisquare analyses adjusted for small sample size and one degree of freedom. Chi-square values were also summed across populations and the total values, with their respective degrees of freedom, were tested for significance (Everitt 1977).
Results and Discussion Screening of 77 SCI genotypes and 200 F1 progeny derived from five reciprocal SCI 3 RDW crosses with either picloram or clopyralid resulted in 100% mortality (data not shown). F1 progeny were susceptible irrespective of whether the pollen donor was SCI or RDW, indicating that the resistant phenotype was recessive in nature and was of nuclear rather than cytoplasmic origin. Although the resistant biotype, which gave rise to the RDW population, had undergone selection for resistance to picloram, the RDW population is more than 350% more resistant to clopyralid compared to picloram based on the resistance ratios to the two herbicides (Fuerst et al. 1996). Because of the superior resistance the RDW biotype exhibits toward clopyralid, we emphasized clopyralid screening over picloram screening to determine the inheritance of pyridinecarboxylic acid resistance. Segregation relationships within reciprocal crosses of F1, pseudo-F2, and testcross populations were not significantly different (data not shown); therefore the reciprocal cross data were pooled within each cross and herbicide treatment.
Brief Communications Table 1. Segregation for resistance to picloram among F1, pseudo-F2, and testcross progeny of yellow starthistle and its goodness-of-fit to a single gene model assuming disomic inheritance Observed ratio (number of seedlings) Population
Cross
Susceptible
Resistant
Expected ratio (S:R)
v2
SCI4 3 RDW4a SCI7 3 RDW7 S4R4F1 3 S7R7F1 R4S4F1 3 R7S7F1 S4R4F1 3 RDW S7R7F1 3 RDW
F1 F1 Pseudo-F2 Pseudo-F2 Testcross Testcross
43 47 129 NAd NA 13
0 0 40 NA NA 21
4:0 4:0 3:1 3:1 1:1 1:1
—b — 0.10c NA NA 1.44c
Total df P a
Reciprocal cross data pooled.
b
Not subjected to analysis because of zero expectation for one class.
1.54 2 0.25–0.5
c
Not significant at P . .10. d NA, not available because cross failed to set sufficient seed.
Limited quantities of seed prevented the screening of some populations with picloram. However, for those pseudo-F2 and testcross populations evaluated, the segregation data indicated that resistance to picloram was controlled by a single recessive allele (Table 1). Screening of pseudo-F2 progeny derived from susceptible mothers (i.e., S4R4F1 3 S7R7F1) with clopyralid indicated close agreement to a classic 3:1 (S:R) ratio for a trait controlled by a single recessive allele (Table 2). Similar results were obtained from the pseudo-F2 progeny derived from resistant mothers (i.e., R4S4F1 3 R7S7F7). The pseudo-F2 segregation data did not fit either of the digenic models involving duplicate dominant epistasis (15S:1R, P . .005), or duplicate recessive epistasis (9S:7R, P . .005). Testcrosses between F1 genotypes and SCI testers produced only susceptible offspring. Crosses between the F1 genotypes and RDW testers produced offspring that segregated 1:1 (S:R) (Table 2). These results provide additional support that herbicide resistance is controlled by a single recessive allele. The lack of segregation among F1 and testcross progeny of SCI genotypes indicates that the SCI parents and testers were homozygous for the clopyralid-susceptible phenotype. Progeny derived from five independent crosses between seven randomly chosen RDW genotypes also did not segregate (i.e., all were resistant; data not shown), indicating that they were allelic and homozygous for the clopyralidresistant phenotype. We have previously shown that picloram resistance (Fuerst et al. 1996) and clopyralid cross-resistance (Valenzuela et al. 2001) in yellow starthistle are not due to differences in foliar absorption, translocation, or metabolism of either herbicide. Although ethylene production was induced in picloram-susceptible (SCI) but not in resistant (RDW) accessions, this response was determined not to be a major factor in resistance to either picloram (Sabba et al. 1998) or clopyralid (Valenzuela et al. 2002). It is possible that resistance
involves a change in the binding site for the pyridinecarboxylic herbicides, which may be the auxin receptor. Despite the fact that auxinic herbicides have been used commercially for more than 50 years, there have been relatively few reports of resistance in weed populations. There are 20 weedy species other than yellow starthistle that have been reported to include populations with acquired resistance to one or more auxinic herbicides, including Canada thistle (Cirsium arvense) (Hodgson 1970), common chickweed (Stellaria media) (Barnwell et al. 1989), field bindweed (Convolvulus arvensis) (Whitworth 1964), gooseweed (Sphenoclea zeylanica) (Itoh and Ito 1994), musk thistle (Carduus nutans) (Bourdot et al. 1989), tall buttercup (Ranunculus acris) (Bourdot et al. 1989), and wild mustard (Sinapis arvensis) (Hall and Romano 1995). Of these, there are only two reports of resistance to pyridinecarboxylic acids, one involving carpet burweed (Soliva sessilis) (Harrington et al. 2001) and one involving wild mustard (Sinapis arvensis) (Hall and Romano 1995). The only report on the basis of inheritance of resistance to an auxinic herbicide was for dicamba resistance in wild mustard. In this case, resistance was determined to be conveyed by a single, dominant, nuclear gene (Jasieniuk et al. 1995). In general, it is unusual for herbicide resistance to be conferred by a recessive gene, and for the vast majority of cases where the inheritance of herbicide resistance has been examined, the resistance trait was found to be conferred by a dominant gene (Jasieniuk et al. 1996). This finding can be explained by the fact that the frequency of a dominant allele can increase in a population much faster than a recessive allele, especially in outcrossing populations (Jasieniuk et al. 1996). There are two recently reported exceptions in which herbicide resistance was shown to be conveyed by a recessive gene or genes: trifluralin resistance in green foxtail (Setaria viridis) (Jasieniuk et al. 1994) and triallate resistance in wild oats (Avena fatua) (Kern et al. 2002). A single nuclear gene was found to convey the trifluralin resistance trait in green
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Journal of Heredity 2003:94(6) Table 2. Segregation for resistance to clopyralid among F1, pseudo-F2, and testcross progeny of yellow starthistle and its goodness-of-fit to a single gene model assuming disomic inheritance Observed ratio (number of seedlings) Population
Cross
Susceptible
Resistant
Expected ratio (S:R)
v2
SCI4 3 RDW4a SCI7 3 RDW7 S4R4F1 3 S7R7F1 R4S4F1 3 R7S7F1 S4R4F1 3 RDW S7R7F1 3 RDW R4S4F1 3 RDW R7S7F1 3 RDW S4R4F1 3 SCI S7R7F1 3 SCI R4S4F1 3 SCI R7S7F1 3 SCI
F1 F1 Pseudo-F2 Pseudo-F2 Testcross Testcross Testcross Testcross Testcross Testcross Testcross Testcross
40 40 178 84 NAd 19 4 4 236 151 105 101
0 0 72 24 NA 23 0 2 0 0 0 0
4:0 4:0 3:1 3:1 1:1 1:1 1:1 1:1 4:0 4:0 4:0 4:0
—b — 1.73c 0.31c NA 0.21c 2.25c 0.17c — — — —
Total df P a
4.72 5 0.5–0.25
Reciprocal cross data pooled.
b
Not subjected to analysis because of zero expectation for one class.
c
Not significant at P . .10.
d
NA, not available because cross failed to set sufficient seed.
foxtail. The two explanations offered by Jasieniuk et al. (1994) for the establishment of a recessive resistance trait in green foxtail was that the species has a high selfing rate and is a prolific seed producer. In the case of triallate resistance in wild oats, resistance was found to be imparted by two recessive nuclear genes. The authors accounted for this result by explaining that wild oats has a high selfing rate, and that the population they studied was under intensive selective pressure for the evolution of resistance (Kern et al. 2002). Our research indicates that picloram resistance and clopyralid cross-resistance in yellow starthistle is conveyed by a single recessive nuclear gene. Given that yellow starthistle is primarily an outbreeder with low self-fertility (Maddox et al. 1996; Sun and Ritland 1998), the establishment of a recessive phenotype would seem unlikely. This is because a mutant recessive allele would confer no immediate selective advantage in the original heterozygote and would be subject to elimination by herbicide application. Selective advantage would occur only in the homozygous recessive condition, which could arise via self-pollination or through the intermating of half-sib progeny derived from a heterozygous genotype. Even though the selfing rate for yellow starthistle was low in our study (less than 0.1%), this level of selffertility is sufficient to generate homozygous-resistant biotypes. For example, only 17 selfed progeny need be produced from a heterozygous parent to have a 99% chance of recovering at least one resistant progeny (Sedcole 1977). Once a resistant biotype was established, it could rapidly propagate because yellow starthistle is a very prolific seed producer. Preliminary results indicate that self- and openpollinated progeny of RDW genotypes demonstrated little or no inbreeding depression (data not shown). Also the
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resistant biotype (RDW) has a similar competitive ability compared to the wild type (SCI), which suggests that there would be no disadvantage for the RDW biotype when there was no herbicide present (Sterling et al. 2001). This information, in conjunction with the fact that the Dayton, WA, population was under intensive and consistent picloram selective pressure for 10 years (Shirman R, personal communication), could explain how the RDW biotype evolved. A recessive trait for picloram resistance and clopyralid cross-resistance is consistent with the observation that resistance has not spread from where it was first observed (Schirman R, personal communication). Due to its recessive nature, the resistance allele is less likely to increase in frequency in yellow starthistle populations that are not under strong selection pressure. Therefore development of picloram resistance in this biotype does not appear to impose a large threat to successful yellow starthistle management as long as auxinic herbicide management is carried out prudently.
Acknowledgments Special thanks to Dr. Pat Fuerst and Roland Schirman, Washington State University, who generously supplied both parent seed types. Also many thanks for the technical assistance provided by Kevin Branum, Erin McHale, Maureen Keegan, and Amber D. Vallotton. This research was supported by the New Mexico State Agricultural Experiment Station, New Mexico State University, the USDA/CSREES National Research Initiative Competitive Grants Program (agreement no. 92-37303-7863), and by a grant from Dow AgroSciences.
Brief Communications
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Received December 5, 2002 Accepted August 30, 2003
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Corresponding Editor: Sally Mackenzie
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