In a range of trematode species, specific members of the parthenitae colony infecting the molluscan first intermediate host appear specialized for defence against co-infecting species. The evolution of such division of labour requires that co-infection entails fitness costs. Yet, this premise has very rarely been tested in species showing division of labour. Using Himasthla elongata (Himasthlidae) and Renicola roscovita (Renicolidae) infecting periwinkles Littorina littorea as study system, we show that the size of emerged cercariae is markedly reduced in both parasite species when competing over host resources. Cercarial longevity, on the other hand, is negatively influenced by competition only in R. roscovita. Season, which may impact the nutritional state of the host, also affects cercarial size, but only in H. elongata. Hence, our study underlines that cercarial quality is, indeed, compromised by competition, not only in the inferior R. roscovita (no division of labour) but also in the competitively superior H. elongata (division of labour).

p53, DNA damage regulated gene (PDRG) and apoptosis-stimulating p53 protein 2 (ASPP2) are p53-related genes which can promote apoptosis. The full-length cDNA sequence of the Px-pdrg and Px-aspp2 genes were characterized and their mRNA expression dynamics under heat stress were studied in diamondback moth (DBM) Plutella xylostella collected from Fuzhou, China. The full-length cDNA of Px-pdrg and Px-aspp2 spans 721 and 4201 bp, containing 395 and 3216 bp of the open reading frame, which encode a putative protein comprising 130 and 1072 amino acids with a calculated molecular weight of 14.58 and 118.91 kDa, respectively. As compared to 25°C, both Px-pdrg and Px-aspp2 were upregulated in chlorpyrifos-resistant (Rc) and -susceptible (Sm) strains of DBM adults and pupae under heat stress. In addition, Rc DBM showed a significantly higher expression level of Px-pdrg and Px-aspp2 in contrast to Sm DBM. The results indicate that high temperature can significantly promote apoptosis process, especially in Rc-DBM. Significant fitness cost in Rc-DBM might be associated with drastically higher transcript abundance of Px-pdrg and Px-aspp2 under the heat stress.

Barnyardgrass biotypes resistant (R) to propanil (PR) or clomazone (CR) have been confirmed in rice production systems in Arkansas. However, it is not clear whether resistance to these herbicides impose any fitness cost on the R biotypes compared to susceptible barnyardgrass (S). The overall objective of this experiment was to determine if the growth and competitiveness of barnyardgrass is altered by resistance to propanil or clomazone and to establish a competitive hierarchy among the S, PR, and CR biotypes. A replacement series study was conducted in a greenhouse using five proportions of S and R biotypes (0 : 100, 25 : 75, 50 : 50, 75 : 25, and 100 : 0). The study was carried out in a completely randomized design (CRD) with four replications. The variables, including plant height, number of tillers, number of leaves, and shoot dry weight, were used for quantifying the differences in competitive abilities. Replacement series indices were calculated to explore the competitiveness. Expected (He) and observed (Ho) values for relative yield (RY) and relative yield total (RYT) were compared for number of tillers, number of leaves, and shoot dry weight for each biotype comparison. Other replacement series indices including competitive ratio (CR), relative crowding coefficient (RCC), and aggressiveness index (AI) also were calculated for these variables. The results showed that there were no major differences among the S and R biotypes for these variables, indicating that in the absence of selection pressure, resistance to propanil or clomazone does not influence the growth and competitiveness of barnyardgrass. The findings will be useful for predicting the dynamics of resistant populations in the absence of herbicide selection and for designing suitable management strategies.

Traditional breeding has been used to develop grain sorghum germplasm that is tolerant to acetolactate synthase (ALS)-inhibiting herbicides (Inzen Technology, DuPont). Inzen sorghum carries a double mutation in the ALS gene (Val560Ile and Trp574Leu), which confers high level of tolerance to ALS-inhibiting herbicides. Overreliance on ALS-inhibiting herbicides for weed control during the 1990s resulted in the evolution of ALS inhibitor–resistant shattercane populations in Nebraska. According to a survey conducted in 2013, ALS inhibitor–resistant weedy Sorghum populations persist in Nebraska. The objectives of this research were to determine whether the ALS mutations present in Inzen sorghum were present in the ALS inhibitor–resistant shattercane and johnsongrass populations detected in Nebraska and northern Kansas, and whether these populations evolved ALS resistance independently. Primers specific to the Val560 and Trp574 region of the ALS gene were used to screen the populations with PCR. The Trp574Leu mutation was present in one ALS inhibitor–resistant johnsongrass population. The Val560Ile was detected in three ALS inhibitor–resistant shattercane, one susceptible shattercane, one ALS inhibitor–resistant johnsongrass, and one susceptible johnsongrass population. Moreover, Val560Ile was present in resistant and/or susceptible individuals within johnsongrass and shattercane populations that were segregating for ALS resistance, indicating that by itself the Val560Ile mutation does not confer resistance to ALS-inhibiting herbicides. None of the populations presented both mutations simultaneously, as does Inzen sorghum. A shattercane population containing the Ser653Thr mutation was also detected. This research indicates that the ALS mutations present in Inzen sorghum already exist individually in weedy sorghum populations. Moreover, our results present strong evidence that ALS resistance in these populations evolved independently. Thus, widespread overreliance on ALS-inhibiting herbicides prior to adoption of glyphosate-tolerant crops in the 1990s exerted sufficient selective pressure on shattercane and johnsongrass populations for resistance to evolve multiple times in the Midwest. Finally, a survey of the 5′ portion of the ALS gene in more diverse wild and weedy Sorghum species was hampered by limited coverage in genomic resequencing surveys, suggesting that refined PCR-based methods will be needed to assess SNP variation in this gene region, which includes the Ala122, Pro197, and Ala205 codons known to confer ALS resistance in other species.

Auxinic herbicides are widely used for control of broadleaf weeds in cereal crops and turfgrass. These herbicides are structurally similar to the natural plant hormone auxin, and induce several of the same physiological and biochemical responses at low concentrations. After several decades of research to understand the auxin signal transduction pathway, the receptors for auxin binding and resultant biochemical and physiological responses have recently been discovered in plants. However, the precise mode of action for the auxinic herbicides is not completely understood despite their extensive use in agriculture for over six decades. Auxinic herbicide-resistant weed biotypes offer excellent model species for uncovering the mode of action as well as resistance to these compounds. Compared with other herbicide families, the incidence of resistance to auxinic herbicides is relatively low, with only 29 auxinic herbicide-resistant weed species discovered to date. The relatively low incidence of resistance to auxinic herbicides has been attributed to the presence of rare alleles imparting resistance in natural weed populations, the potential for fitness penalties due to mutations conferring resistance in weeds, and the complex mode of action of auxinic herbicides in sensitive dicot plants. This review discusses recent advances in the auxin signal transduction pathway and its relation to auxinic herbicide mode of action. Furthermore, comprehensive information about the genetics and inheritance of auxinic herbicide resistance and case studies examining mechanisms of resistance in auxinic herbicide-resistant broadleaf weed biotypes are provided. Within the context of recent findings pertaining to auxin biology and mechanisms of resistance to auxinic herbicides, agronomic implications of the evolution of resistance to these herbicides are discussed in light of new auxinic herbicide-resistant crops that will be commercialized in the near future.

Insecticide resistance is frequently associated with fitness disadvantages in the absence of insecticides. However, intense past selection with insecticides may allow the evolution of fitness modifier alleles that mitigate the cost of insecticide resistance and their consequent fitness disadvantages. Populations of Sitophilus zeamais with different levels of susceptibility to insecticides show differences in the accumulation and mobilization of energy reserves. These differences may allow S. zeamais to better withstand toxic compounds without reducing the beetles' reproductive fitness. Enzymatic assays with carbohydrate- and lipid-metabolizing enzymes were, therefore, carried out to test this hypothesis. Activity levels of trehalase, glycogen phosphorylase, lipase, glycosidase and amylase were determined in two insecticide-resistant populations showing (resistant cost) or not showing (resistant no-cost) associated fitness cost, and in an insecticide-susceptible population. Respirometry bioassays were also carried out with these weevil populations. The resistant no-cost population showed significantly higher body mass and respiration rate than the other two populations, which were similar. No significant differences in glycogen phosphorylase and glycosidase were observed among the populations. Among the enzymes studied, trehalase and lipase showed higher activity in the resistant cost population. The results obtained in the assays with amylase also indicate significant differences in activity among the populations, but with higher activity in the resistant no-cost population. The inverse activity trends of lipases and amylases in both resistant populations, one showing fitness disadvantage without insecticide exposure and the other not showing it, may underlay the mitigation of insecticide resistance physiological costs observed in the resistant no-cost population. The higher amylase activity observed in the resistant no-cost population may favor energy storage, preventing potential trade-offs between insecticide resistance mechanisms and basic physiological processes in this population, unlike what seems to take place in the resistant cost population.

Fitness costs associated with resistance genes expressed in the absence of insecticides affect the evolution of insecticide resistance and the outcome of resistance management programmes. However, measurements of fitness costs may not be straightforward as they vary with environmental conditions. The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), has developed resistance to spinosad, the first insecticide of the Naturalyte class, after only a few years of field application of this product. In this study, we compared the performance of two homogenous strains of P. xylostella, one susceptible (SS) and the other resistant (RR) to spinosad at an unfavourable, low natural temperature regime, a favourable median-fluctuating temperature regime and an unfavourable high-fluctuating temperature regime. The RR strain showed only marginal fitness cost at the median temperature regime. At the low temperature regime, the RR strain failed to produce any viable offspring, while the SS strain achieved positive population growth. At the high temperature regime, the RR strain showed a 33% decrease in intrinsic rate of increase compared to the SS strain. The results demonstrate that fitness costs of resistance to spinosad are temperature-dependent, increasing in scale at unfavourably low and high temperatures; costs were particularly high at low temperatures. Suggestions for designing effective management programmes are made to delay or avoid development of resistance to spinosad by P. xylostella under different temperature conditions.

Costs of sexual traits are of central importance to the theory of sexual selection. To qualify as a cost in line with theoretical models, empirical studies must demonstrate that sexual traits cause negative effects on one component of fitness of the trait bearer. Moreover, it must be demonstrated that the costs are differential such that negative effects on fitness are more severe for individuals in poor condition than for individuals in good condition. However, in the current literature, there is confusion over what qualifies as a cost, and costs are often anticipated based on findings of increased expenditure. Consequently, it seems that the generally accepted notion that sexual traits are costly is in fact based almost exclusively on indirect evidence and that direct empirical evidence is very scarce.