Dawn M. Higginson

Three cadherin alleles associated with resistance to Bacillus thuringiensis in pink bollworm

Evolution of resistance by pests is the main threat to long-term insect control by transgenic crops that produce Bacillus thuringiensis (Bt) toxins. Because inheritance of resistance to the Bt toxins in transgenic crops is typically recessive, DNA-based screening for resistance alleles in heterozygotes is potentially much more efficient than detection of resistant homozygotes with bioassays. Such screening, however, requires knowledge of the resistance alleles in field populations of pests that are associated with survival on Bt crops. Here we report that field populations of pink bollworm (Pectinophora gossypiella), a major cotton pest, harbored three mutant alleles of a cadherin-encoding gene linked with resistance to Bt toxin Cry1Ac and survival on transgenic Bt cotton. Each of the three resistance alleles has a deletion expected to eliminate at least eight amino acids upstream of the putative toxin-binding region of the cadherin protein. Larvae with two resistance alleles in any combination were resistant, whereas those with one or none were susceptible to Cry1Ac. Together with previous evidence, the results reported here identify the cadherin gene as a leading target for DNA-based screening of resistance to Bt crops in lepidopteran pests.

Effects of Gossypol on Fitness Costs Associated with Resistance to Bt Cotton in Pink Bollworm

Abstract Fitness costs associated with insect resistance to Bacillus thuringiensis (Bt) crops may help to delay or prevent the spread of resistance alleles, especially when refuges of non-Bt host plants are present. The potential for such delays increases as the magnitude and dominance of fitness costs increase. Here, we examined the idea that plant secondary chemicals affect expression of fitness costs associated with resistance to Bt cotton in Pectinophora gossypiella (Saunders). Specifically, we tested the hypotheses that gossypol affects the magnitude or dominance of fitness costs, by measuring performance of three independent sets of pink bollworm populations fed artificial diet with and without gossypol. Each set had an unselected susceptible population, a resistant population derived by selection from the susceptible population, and the F1 progeny of the susceptible and resistant populations. No individuals completed development on diets with gossypol in one set, suggesting that these individuals partially lost the ability to detoxify this chemical. In the other two sets, costs affecting survival did not support the hypotheses, but costs affecting pupal weight did. Adding gossypol to diet increased the magnitude and dominance of costs affecting pupal weight. In one of the two sets with survivors on diet with gossypol, costs affecting development time were less recessive when gossypol was present in diet. These results indicate that gossypol increased the magnitude and dominance of some fitness costs. Better understanding of the effects of natural plant defenses on fitness costs could improve our ability to design refuges for managing insect resistance to Bt crops.

Association Between Resistance to Bt Cotton and Cadherin Genotype in Pink Bollworm

Abstract Two strains of pink bollworm, Pectinophora gossypiella (Saunders), each derived in 1997 from a different field population, were selected for resistance to Bacillus thuringiensis (Bt) toxin Cry1Ac in the laboratory. One strain (MOV97-R) originated from Mohave Valley in western Arizona; the other strain (SAF97-R) was from Safford in eastern Arizona. Relative to a susceptible laboratory strain, Cry1Ac resistance ratios were 1700 for MOV97-R and 520 for SAF97-R. For the two resistant strains, larval survival did not differ between non-Bt cotton and transgenic cotton producing Cry1Ac. In contrast, larval survival on Bt cotton was 0% for the two unselected parent strains from which the resistant strains were derived. Previously identified resistance (r) alleles of a cadherin gene (BtR) occurred in both resistant strains: r1 and r3 in MOV97-R, and r1 and r2 in SAF97-R. The frequency of individuals carrying two r alleles (rr) was 1.0 in the two resistant strains and 0.02 in each of the two unselected parent strains. Furthermore, in two hybrid strains with a mixture of susceptible (s) and r alleles at the BtR locus, all survivors on Bt cotton had two r alleles. The results show that resistance to Cry1Ac-producing Bt cotton is associated with recessive r alleles at the BtR locus in the strains of pink bollworm tested here. In conjunction with previous results from two other Bt-resistant strains of pink bollworm (APHIS-98R and AZP-R), results reported here identify the cadherin locus as the leading candidate for molecular monitoring of pink bollworm resistance to Bt cotton.

Female reproductive tract form drives the evolution of complex sperm morphology

The coevolution of female mate preferences and exaggerated male traits is a fundamental prediction of many sexual selection models, but has largely defied testing due to the challenges of quantifying the sensory and cognitive bases of female preferences. We overcome this difficulty by focusing on postcopulatory sexual selection, where readily quantifiable female reproductive tract structures are capable of biasing paternity in favor of preferred sperm morphologies and thus represent a proximate mechanism of female mate choice when ejaculates from multiple males overlap within the tract. Here, we use phylogenetically controlled generalized least squares and logistic regression to test whether the evolution of female reproductive tract design might have driven the evolution of complex, multivariate sperm form in a family of aquatic beetles. The results indicate that female reproductive tracts have undergone extensive diversification in diving beetles, with remodeling of size and shape of several organs and structures being significantly associated with changes in sperm size, head shape, gains/losses of conjugation and conjugate size. Further, results of Bayesian analyses suggest that the loss of sperm conjugation is driven by elongation of the female reproductive tract. Behavioral and ultrastructural examination of sperm conjugates stored in the female tract indicates that conjugates anchor in optimal positions for fertilization. The results underscore the importance of postcopulatory sexual selection as an agent of diversification.

Evolutionary trade-offs of insect resistance to Bacillus thuringiensis crops: Fitness cost affecting paternity

Abstract Evolution of resistance to insecticides provides a useful model for examining fitness trade‐offs associated with adaptation to stress. Here, we examined male reproductive costs in pink bollworm (Pectinophora gossypiella) resistant to an insecticidal protein of Bacillus thuringiensis (Bt) produced by transgenic cotton, using contrasts between two pairs of related susceptible and resistant strains. Without competition for access to females, no costs affecting reproductive success of resistant males were observed. Resistant and susceptible males had similar mating frequency and fertility. Additionally, fecundity of females mated to resistant and susceptible males was comparable. In competition for matings with virgin females, resistant and susceptible males had comparable success in one strain, whereas susceptible males tended to mate more often than resistant males in the other. However, irrespective of strain origin, resistant males that mated first sired significantly less offspring than susceptible males that mated first. The reduced first‐male paternity in resistant males may involve reduced sperm precedence caused by mutations in a cadherin gene linked with resistance to Bt cotton.

Evolution of intra-ejaculate sperm interactions: do sperm cooperate?

Sperm are often considered to be individuals, in part because of their unique genetic identities produced as a result of synapsis during meiosis, and in part due to their unique ecology, being ejected away from the soma to continue their existence in a foreign environment. Selection at the level of individual sperm has been suggested to explain the evolution of two enigmatic sperm phenotypes: sperm heteromorphism, where more than one type of sperm is produced by a male, and sperm conjugation, where multiple sperm join together for motility and transport through the female reproductive tract before dissociation prior to fertilization. In sperm heteromorphic species, only one of the sperm morphs typically participates in fertilization, with the non‐fertilizing “parasperm” being interpreted as reproductive altruists. Likewise, in species with sperm conjugation, high levels of sperm mortality have been suggested to be required for conjugate break‐up and this has been considered evidence of kin‐selected altruism. However, it is unclear if sperm possess the heritable variation in fitness (i.e. are individuals) required for the evolution of cooperation. We investigate the question of sperm individuality by focusing on how sperm morphology is determined and how sperm conjugates are formed. Concentrating on sperm conjugation, we discuss functional hypotheses for the evolutionary maintenance of this remarkable trait. Additionally, we speculate on the potential origins of sperm heteromorphism and conjugation, and explore the diversification and losses of these traits once they have arisen in a lineage. We find current evidence insufficient to support the concept of sperm control over their form or function. Thus, without additional evidence of haploid selection (i.e. sperm phenotypes that reflect their haploid genome and result in heritable differences in fitness), sperm heteromorphism and conjugation should be interpreted not as cooperation but rather as traits selected at the level of the male, much like other ejaculatory traits such as accessory gland proteins and ejaculate size.

Complex interactions with females and rival males limit the evolution of sperm offence and defence

Postcopulatory sexual selection favours males which are strong offensive and defensive sperm competitors. As a means of identifying component traits comprising each strategy, we used an experimental evolution approach. Separate populations of Drosophila melanogaster were selected for enhanced sperm offence and defence. Despite using a large outbred population and evidence of substantive genetic variation for each strategy, neither trait responded to selection in the two replicates of this experiment. Recent work with fixed chromosome lines of D. melanogaster suggests that complex genotypic interactions between females and competing males contribute to the maintenance of this variation. To determine whether such interactions could explain our lack of response to selection on sperm offence and defence, we quantified sperm precedence across multiple sperm competition bouts using an outbred D. melanogaster population exhibiting continuous genetic variation. Both offensive and defensive sperm competitive abilities were found to be significantly repeatable only across matings involving ejaculates of the same pair of males competing within the same female. These repeatabilities decreased when the rival male stayed the same but the female changed, and they disappeared when both the rival male and the female changed. Our results are discussed with a focus on the complex nature of sperm precedence and the maintenance of genetic variation in ejaculate characteristics.

CONVERGENCE, RECURRENCE AND DIVERSIFICATION OF COMPLEX SPERM TRAITS IN DIVING BEETLES (DYTISCIDAE)

Sperm display remarkable morphological diversity among even closely related species, a pattern that is widely attributed to postcopulatory sexual selection. Surprisingly few studies have used phylogenetic analyses to discern the details of evolutionary diversification in ornaments and armaments subject to sexual selection, and the origins of novel sperm traits and their subsequent modification are particularly poorly understood. Here we investigate sperm evolution in diving beetles (Dytiscidae), revealing dramatic diversification in flagellum length, head shape, presence of sperm heteromorphism, and the presence/type of sperm conjugation, an unusual trait where two or more sperm unite for motility or transport. Sperm conjugation was found to be the ancestral condition in diving beetles, with subsequent diversification into three forms, each exhibiting varying degrees of evolutionary loss, convergence, and recurrence. Sperm head shape, but not length or heteromorphism, was found to evolve in a significantly correlated manner with conjugation, consistent with the different mechanisms of head alignment and binding required for the different forms of conjugation. Our study reveals that sperm morphological evolution is channeled along particular evolutionary pathways (i.e., conjugate form), yet subject to considerable diversification within those pathways through modification in sperm length, head shape, and heteromorphism.

Size-dependent alternative male mating tactics in the yellow dung fly, Scathophaga stercoraria

Whenever males can monopolize females and/or resources used by females, the opportunity for sexual selection will be great. The greater the variation among males in reproductive success, the greater the intensity of selection on less competitive males to gain matings through alternative tactics. In the yellow dung fly, Scathophaga stercoraria, males aggressively compete for access to receptive, gravid females on fresh dung. Larger males are better able to acquire mates and to complete copulation successfully and guard the female throughout oviposition. Here we demonstrate that when an alternative resource is present where females aggregate (i.e. apple pomace, where both sexes come to feed), smaller males will redirect their searching for females from dung to the new substrate. In addition, we identify a class of particularly small males on the alternative substrate that appears never to be present searching for females on or around dung. Smaller males were found to have a mating ‘advantage’ on pomace, in striking contrast to the pattern observed on dung, providing further support for the existence of an alternative male reproductive tactic in this species.

Evolution of long-term coloration trends with biochemically unstable ingredients

The evolutionarily persistent and widespread use of carotenoid pigments in animal coloration contrasts with their biochemical instability. Consequently, evolution of carotenoid-based displays should include mechanisms to accommodate or limit pigment degradation. In birds, this could involve two strategies: (i) evolution of a moult immediately prior to the mating season, enabling the use of particularly fast-degrading carotenoids and (ii) evolution of the ability to stabilize dietary carotenoids through metabolic modification or association with feather keratins. Here, we examine evolutionary lability and transitions between the two strategies across 126 species of birds. We report that species that express mostly unmodified, fast-degrading, carotenoids have pre-breeding moults, and a particularly short time between carotenoid deposition and the subsequent breeding season. Species that expressed mostly slow-degrading carotenoids in their plumage accomplished this through increased metabolic modification of dietary carotenoids, and the selective expression of these slow-degrading compounds. In these species, the timing of moult was not associated with carotenoid composition of plumage displays. Using repeated samples from individuals of one species, we found that metabolic modification of dietary carotenoids significantly slowed their degradation between moult and breeding season. Thus, the most complex and colourful ornamentation is likely the most biochemically stable in birds, and depends less on ecological factors, such as moult timing and migration tendency. We suggest that coevolution of metabolic modification, selective expression and biochemical stability of plumage carotenoids enables the use of unstable pigments in long-term evolutionary trends in plumage coloration.