Wild collections are diverse across geographic regions and highly heterogeneous within populations, including the presence or absence of arcelin . The great variation in their expression of arcelin production, suggests that this seed protein may have a limited effect on fitness. Alternatively, the cost of acrelin production may outweigh the benefits in some settings. This raises the possibility that domestication did not occur in the limited region where arcelin producing accessions are found or by chance during domestication non-arcelin individuals were selected . Insect herbivores have exerted selective pressures on both wild and domesticated plants . For self-pollinating lineages to evolve and be maintained, the fitness cost of inbreeding depression – the difference in fitness of inbred and outcrossed progeny – cannot be too high . Herbivory can increase inbreeding depression, though the interaction between pest and mating system can be complex and varied . Autogamy, or self-pollination, and asexual reproduction have been favored over allogamy, commonly known as out-crossing, blueberry packaging box during domestication . The combination of selection for inbreeding and inbreeding depression as a result of herbivore induced selection has contributed to the trend of increased susceptibility to insect pests of crop plants as compared to their wild relatives.
The accelerating fragmentation or total clearing of wild ecosystems to make way for farmland has resulted in an incalculable loss of genetic diversity . This loss of biodiversity has increased the pressure from insect herbivores on crop plants by several mechanisms, including the loss of predators and parasitoids, as well as genetic erosion and reduced gene flow between crops and their wild relatives. Close correlations between plant and insect diversity have been found in wild and agricultural ecosystems . The habitat needs of specialist herbivores as well as predators and parasitoids may not be met by simplified agricultural landscapes. As a result, the ecosystem services they provide in more diverse landscapes will be lost . Overall, crop domestication and the ecological disturbance of agriculture has exerted a rselection pressure on insect herbivores resulting in the evolution of more serious pests . Within the center of domestication, landscape diversity is a reserve of crop diversity and insect diversity . Loss of genetic diversity within these ecosystems can have several consequences for the future evolution of crops and insect herbivores. Genetic diversity in a population is a prerequisite for evolution by natural and artificial selection . This applies to both crop plants and their insect herbivores . Gene flow from wild relatives is an important source of anti-herbivore traits in crop plants. Hence, genetic erosion of crop gene pools is occurring rapidly due the loss of habitat for wild relatives . Whilemodern breeding efforts have sought to identify and incorporate defensive traits from wild relatives, this is slow work and cannot proceed without conservation .
Additionally, the loss of spatial diversity across agricultural ecosystems counters the ability of balancing selection to maintaining nonneutral genetic variation in crop populations including landraces . In addition to trait maintenance and ease of adaptation to novel circumstances, both intraspecific and interspecific diversity make agricultural landscapes more difficult for herbivores to navigate . In polycultures, insects are less likely to find and stay on their host plants . Additionally, increased niche partitioning in diverse landscapes leads to more efficient capture of available resources. This increases plant productivity in diverse landscapes as compared to monocultures .Polyploidy is common in plants and is especially prevalent in domesticated species . Autopolyploidy is when polyploidy arises from whole genome duplication. Allopolyploidy is when polyploidy arises from the hybridization of two or more whole genomes. Adding copies of the genome can result in new expression and interactions between genes and alleles, thereby affecting anti-herbivore defense traits . Whether a change in ploidy comes through autopolyploidy or allopolyploidy may affect the outcome for anti-herbivore defense traits. In allopolyploids, some evidence indicates that there is expression of novel phenolic compounds and enzymatic diversity . This greater chemical diversity may help combat the overall loss of chemical diversity that results from the initial domestication event and as a result amplify some anti-herbivore traits . However, more research is needed on this subject.
By contrast, autopolyploidy has been found to have varied effects on chemical phenotypes including the expression of novel compounds, the absence of compounds found in diploids, and changes in the regulation of tissue-specific chemical production . There is evidence to support two main scenarios of insect herbivore response to polyploidization. In the first, polyploidy may create new defenses that allow a host plant to escape from herbivory and coevolutionary relationships with its insect pests . An example of this is found in the order Brassicales, which includes many domesticated species. The successful evolution of increasingly complex glucosinolate defense chemicals for protection against the cabbage moth has been driven by gene and genome duplication . Duplicated genes in the glucosinolate pathway were retained at a rate of over 95% after whole genome duplication compared to an average rate of 45% maintenance in duplication overall in protein coding genes . This indicates that polyploidization during domestication may have improved antiherbivore defense mechanisms in some cases . In the second scenario, polyploidy may expand the range of hostplants and their herbivores resulting in greater diversification and ecological opportunities for the pest . This can be seen in the example of gall midges , which have species-specific preferences for – and therefore habitat limitation to – cytotypes in creosote bush, Larrea tridentata and the perennial brassica, Cardamine pratensis . While no examples for this scenario can be found in research on domesticated crops, it is likely that polyploidization during domestication may have altered crop plant and insect herbivore interactions by expanding the ecological range of both crops and insect herbivores. Given the syndrome-like nature of plant defense, the multi-locus control of many individual plant defense traits, the higher load of deleterious mutations, and the gene loss known to occur after polyploidization, it might seem that polyploid crops would be especially sensitive to changes in ploidy and genetic drift during domestication . However, there is strong evidence to suggest that the genes controlling specialized metabolic pathways for certain defense traits are clustered . As a result, polyploidy is, in many cases, an asset to plants for maintaining and evolving anti-herbivore defense mechanisms while undergoing selection during domestication . Additionally, polyploid plants are often clonally propagated – a practice which may maintain advantageous combinations of alleles. Asexual reproduction has been found especially effective in maintaining defenses against specialist herbivores, but the associated loss of chemical diversity may come at a cost of adaptations that defend against generalist herbivores .Selection during domestication can have varied effects on the characteristics of crop plants depending on the harvested organ, mating system, and life history of the plant . Humans have found diverse ways of consuming a wide variety of plant organs, though reproductive organs, like fruits and seeds, are favored staples given their high caloric and nutritional value . The modular nature of plants and organ-specific genetic control of defense trait expression has allowed for the selection of crop plants with defenses that are differentiated in reproductive and harvested organs . Reproductive structures, including flowers, fruits, and seeds, blueberry packaging containers are usually photosynthetic sinks that require nutrient and energy investments from other parts of the plant to build and defend .
For crops in which a reproductive organ is also the harvested organ – such as fruits, grains, pulses, and oil seeds – selection for or against herbivore defense traits may have been imposed by consumer preferences as discussed below. However, for crops in which the harvested organ is not a reproductive organ – such as leafy greens, tubers, and biomass crops – the harvested organ may have lost defenses when resources were allocated to yield rather than defense . The harvested organs of crops have undergone especially intensive selection during domestication . Damage to the harvested organs by insect herbivores can directly affect both the quality and yield of a crop. As a result, selection during domestication may have been more intense for these organs. For edible crops however, selection may have acted against defense traits, as palatability or safety for human consumers was prioritized. This can be seen in the example of cyanogenic glycosides in almonds. Presence of these compounds in the kernels is an effective defense against some insect herbivores , but is also toxic to human consumers . As a result, the trait has been strongly selected against in domesticated almonds . Studies have shown that the pre-cyanogenic compound prunasin is produced outside the kernel and then transported in and stored as amygdalin . While control of prunasin production is multigenic, its presence in kernels is controlled by a single gene . This is like other classic domestication syndrome traits which are controlled by single genes or large effect quantitative trait loci . A similar genetic effect of domestication on defense compound biosynthesis can be seen in the example of cucurbitacin in the Cucurbitaceae family. Cucurbitacin confers a bitter flavor to plant tissue and if consumed in large enough doses it can be fatally toxic to human consumers . While some insects have adapted to tolerate and even benefit from cucurbitacin, it is a strong deterrent to feeding and oviposition to other insect species . However, just as amygdalin storage in almond kernels is controlled by a single gene, a transcription factor Bt controls biosynthesis of cucurbitacin from the Bi locus in fruits and has undergone strong selection during domestication . This demonstrates strong and independent selection during domestication that reduced defense traits in the harvested organ of a crop. In instances in which the antiherbivore trait is not toxic or unpalatable to human consumers, there is evidence that selection during domestication has maintained defensive traits in harvested organs. This is the hypothesis of Rodriguez-Saona et al. for the case of cranberries and selection for anthocyanins. Anthocyanins are a class of compounds known to provide defense against insect herbivores in some cases and are also understood to be beneficial for human consumers . It was found that anthocyanins have not been maintained as an effective defense in North American highbush blueberries . Both highbush blueberries and cranberries were recently domesticated so they are likely still undergoing significant selection in cultivated environments . In some cases, the harvested organs of a crop may have become more susceptible to an insect pest because selection during domestication made them more attractive hosts rather than eliminating defenses. This can be seen in the example of chili peppers, which were domesticated for both ornamental and culinary uses. Culinary peppers are much more susceptible to pepper weevils than their ornamental or wild relatives . This is despite having been selected for both higher and lower levels of the defense compound capsaicin as compared to their wild relatives. The greater susceptibility could instead be due to factors like flower size and pericarp thickness. Similarly, in Lima bean, the larger seed size of domesticated forms has been found to reduce competition among seed beetle larvae, thereby making them more destructive pests .Domestication is primarily thought of as affecting the genes of the domesticated species . However, there is substantial evidence that the genetics of insect herbivores have also been affected by crop evolution prior to, during, and after domestication . This is akin to the situation described for some host-pathogen interactions in wild and domesticated common bean . Insect herbivores feeding on crop plants can form distinct populations from other members of the same species feeding on their wild host plants and may even diverge into distinct species . For example, generalist herbivores that feed on multiple crop plants can form distinctive strains associated with a given crop. This was demonstrated in the case of Fall Army Worm, Spodoptera frugiperda . In this study, it was shown that when strains preferring maize or Bermuda grass were moved to the other crop, the plants expressed different induced defenses than when they were attacked by the strain adapted to the same crop. Since these induced defenses can compromise caterpillar growth, strong selection pressure is being exerted on the composition of their saliva to minimize plant defense trait selection .