The complementation test and its exceptions

In order to understand genetic interactions, one mechanism is to determine whether the two genes are in the same or different complementation groups. However, this can be tricky because there are exceptions to the rules.

This review summarizes the theory behind the complementation test and its applications by citing examples from working with C. elegans. The criteria for the complementation test are discussed: the mutations must be recessive, mutations that fail to complement can cause homozygous phenotypes, the heterozygote can express a more severe phenotype than either homozygote alone, and if the gene is in a complex locus, the mutation can affect more than one gene product. This article discusses complications with the complementation test where it is unreliable and in certain cases, may even be misleading by providing examples. One situation is when alleles of the same gene complement each other, defined as intragenic complementation. The author also provides examples of different ways in which mutant alleles that fail to complement can mutually correct each other by reducing the dosage of a mutant product, stabilizing a complex, or by providing the missing function. Another situation that may be misleading is when there is second-site non- complementation in which alleles in the same complementation groups behave as if they are alleles that are non- complementary by acting as poisons or by double-haplo-insufficiency. This article does a good job of providing examples of different scenarios that were covered in lecture.

The article goes into more detail about second-site non-complementation using mutations in the spe-6 gene of C. elegans as an example. Mutations in the spe-6 gene leads to MSP assembly defect, a cytoskeletal protein which provides motility for spermatozoa. This leads to sterility due to defective primary spermatocytes that do not form spermatids. Spe-6 mutant spermatocytes arrest meiosis at diakinesis and chromosomes fail to segregate to the metaphase plate resulting in spermatocytes with four half-spindles surrounding unsegregated chromosomes. All four spe-6 alleles plus a chromosome III deficiency that deletes the spe-6 gene fail to complement two small overlapping chromosome IV deficiencies, eDf18 and eDf19. The non-complementation can be expected if gene products coded byeDf18 and eDf19 are necessary with the spe-6 gene product to promote MSP assembly. Varkey and others tested all spe-6 alleles located on chromosome III against both deficiencies, eDf18 and eDf19 of chromosome IV. All the other alleles failed to complement either deficiency. However, the spe-6(hc143) allele complemented better than the others. Varkey then tested another deficiency of thespe-6 gene, eDf2(III), which deletes the entire spe-6 gene. Results indicated that it also failed to complement leading to the conclusions that the interaction between the spe-6 gene and the unlinked deficiencies is due to reduced dosage of hemizygous genes.

References :
Yook, K. Complementation (October 06, 2005), WormBook, ed. The C. elegans Research Community, WormBook, doi/10.1895/wormbook.1.24.1, http://www.wormbook.org.
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=wormbook&part=complementation
J. P. Varkey, P. L. Jansma, A. N. Minniti, and S. Ward. The Caenorhabditis Elegans Spe-6 Gene Is Required for Major Sperm Protein Assembly and Shows Second Site Non-Complemehttp://www.ncbi.nlm.nih.gov/pmc/articles/PMC1205300/?tool=pubmedntation with an Unlinked Deficiency. Genetics. 1993 January; 133(1): 79-86. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1205300/?tool=pubmed