[Ishani7]

Plant pests range from nematodes to birds and mammals. Most pests of crops important for humans are insects belonging to Lepidoptera, Diptera, Coleoptera etc. Genetic manipulation has for and against points in recent research which requires further studies on development of invasive plants, genetically improved pests etc.

Bt approach is one example where genetic manipulation is used for pest resistance. Genes encoding for the cry proteins with pesticide activity of Bacillus thuringiensis bacterium is incorporated into plant genomes as in cotton. Bt genes in plant genome produces cry proteins and develop a self-defense mechanism against pests. Bt genes produce number of toxins which affect insect larvae. These cry proteins are also known as Insecticidal crystal proteins (ICP) which is an endotoxin produced by bacteria during sporulation. Genes encoding for ICP’s are carried on bacterial plasmids which is a superfamily of related genes. Cry proteins are different in size to one another, but still share a common active core. Active core of Cry A gene has 3 domains; which were specialized for creating pores through membrane of the insect gut, receptor recognition and for protection from degradation in the protein product. When the cry proteins are ingested by insect larvae, the protein crystals are solubilized in mid gut. Larger cry proteins are proteolytically cleaved to release the active fragment of protein. This interacts with high affinity receptors in the mid gut brush border membrane. This binding results in opening of cation selective pores in the membrane. The flow of the cations into the cells results in osmotic lysis of mid gut epithelial cells. Conditions prevailing in mid gut of larvae result in activation of specific cry proteins. Among cry proteins, δ endotoxins are extremely toxic at low concentrations. Isolated crystal proteins in mass scale are used as biopesticides. Appearance of resistant pests, altered interactions between plants and environment are widely discussed issues on Bt crops. It was found that pollen grains of Bt maize were toxic to Monarch butterfly, which may interfere with environmental interactions.

Genetic control of pests through reciprocal translocation is another advanced method. Reciprocal translocation leads to reduction in fertility, population displacement and genetic transformation with a conditional lethal trait. This involves reducing or nullifying the fertility of pest species using genetic changes.

In reciprocal translocation, broken pieces of two non-homologous chromosomes swap. If the break occurs without damaging a critical area of the genome, the organism will have the full complement of original gamete information and can appear perfectly normal. It is the production of gametes where the genetic abnormality manifests itself. When normal diploid cells undergo meiosis, homologous pairs of chromosomes line up with each other during Metaphase 1. But when there is a reciprocal translocation, the homologous pairs of the original chromosomes line up with one another making a peculiar pattern at Metaphase 1. Normally during Anaphase 1, homologous chromosomes migrate to opposite poles, pulled by spindle apparatus. But with a reciprocal translocation, there are there are three possible ways for the chromosomes to sort themselves to migrate to opposite poles. This reduction division can produce six different gametes. Theoretically, fertility of the translocation heterozygote would be reduced to one third of the normal. This can occur naturally and spontaneously, but frequency can be increased by low doses of ionizing radiation. To make necessary genetic changes; radiation is used. It is evident that not all insect species can tolerate the radiation levels necessary to make them sterile. Practically, probabilities of possible pairings of chromosomes are not equal. So fertility is 1/3 to 1/2 of normal. If more than one translocation occurs, fertility would be reduced more. The possible crosses between viable gametes of translocation heterozygote would produce a normal diploid, a translocation heterozygote and a translocation homozygote (1:2:1). The impact of reduced fertility of the translocation heterozygote on the overall population will depend on the species involved and its environment. Multiple translocations are used to create a strain that is genetically identical to wild type except that the order of genes on chromosomes scrambled. The wild type and the translocated strain are equally viable and fertile. But the hybrid is viable but not fertile. This is used in the phenomenon of population displacement.

Inherited sterility is an approach to the genetic manipulation of pest population in which released insects are fertile, but their progeny will be infertile. This is known as delayed sterility. Cytoplasmic incompatibility and multiple polyploidy also can be used to introduce genetic alterations to produce infertility.