Caenorhabditis heterochronic genes: conservation and divergence of developmental roles.

2020-04-20T22:54:10Z (GMT) by Maria Ivanova Eric G. Moss
Heterochronic genes control the timing of developmental events during four C. elegans larval stages. They encode for proteins and miRNA that control them. Mutations in heterochronic genes can cause skipping or reiteration of cell fates associated with certain larval stages (L1 through L4).

lin-28 and lin-46 are two well-studied heterochronic genes.

lin-28 acts early in larval development and controls what happens in many tissues during the L2 stage: if it is missing or down-regulated too soon, L2-specific events are skipped. If it is not down-regulated on time, L2-specific events are repeated. This seems to be the sole function of lin-28 in C. elegans.

lin-46 works at the same time and controls L2 events in the opposite way: When lin-46 is missing, L2-specific events are sometimes repeated. Interestingly, a lin-28; lin-46 double null mutant, where both genes are inactive, develops nearly perfectly normally.

An important question is the conservation of roles of the heterochronic genes in the evolution. lin-28 homologs were studied in the fruit fly, clawed frog, zebrafish, mouse, and human. lin-28 is associated with the early developmental stages and undifferentiated cells and downregulated on the way to the differentiation.

LIN-28 in C. elegans and C. briggsae has an 89% identity with the accumulation of mutations within the first 80 amino acids.

The aim was to create mutations in lin-28 in lin-46 of C. briggsae and determine (1) whether they produce the same phenotypes as in C. elegans, and (2) determine whether they genetically suppress each other, as in C. elegans

Several mutant alleles were created by injecting worms with Cas9 protein and in-vitro transcribed guide RNAs.
Two frameshift alleles of Cb-lin-28 which are likely nulls. One 3’UTR deletion of Cb-lin-28 was obtained which likely affects a miRNA binding site, causing a gain-of-function (gf) allele. Two frameshift mutation of Cb-lin-46 were obtained, which are also likely nulls.