To review:
On the basis of localization of Macho mRNA in the tunicate embryo, establishment of distribution of a specific mRNA from cell biological perspective.
Introduction:
Conklin demonstrated differing coloration of cytoplasmic regions of tunicate embryo Styela partita to follow each of the cells of the tunicate embryo to its fate in the larva. Conklin found that cells receiving clear cytoplasm become ectoderm; those containing yellow cytoplasm give rise to mesodermal cells; those that incorporate slate-gray inclusions become endoderm; and light gray cells become the neural tube and notochord.
J.R. Whittaker in 1973 conducted an experiment on the cytoplasmic segregation of tissue determinants in early tunicate embryos. He found that when the pair of B4.1 blastomeres were removed and kept in isolation gave rise to muscle tissues while no other blastomeres were able to give rise to muscle tissue. The yellow-pigmented cytoplasm contains mRNA (messenger ribonucleic acid) for a muscle-specific transcription factor known as Macho. The blastomeres containing region of yellow cytoplasm give rise to muscle cells.
The primary function of Macho mRNA is development of tail muscle in Styela; removal of Macho mRNA results in the loss of tail muscle differentiation of the B4.1 blastomeres, the microinjection of Macho mRNA into different blastomeres promotes ectopic muscle division.
Therefore, it is concluded that Macho mRNA plays essential role in development of tail muscles of tunicates by retaining Macho mRNA from the egg cytoplasm with each round of mitosis.