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  • Orchid flower ABCs : the science behind flower development and pelorics

    Hello all, there seem to be many people interested in peloric orchids. As a PhD. student whose work deals with genes responsible for the development of flower, I thought it would be nice to share the science behind orchid flower development and specially the development of peloric nature. So here it goes for those of you interested.

    Almost all living things run with the same genetic code, all animals and plants have hundreds if not thousands of genes. But not all genes are expressed everywhere in all the cells. It is this differential expression that gives rise to the different organs like the leaves, stems, roots and flowers, i.e even though all cells have all the genes, only the genes specific to that particular organ are expressed in that cell, the other genes are in a silent resting state.

    In flowers a group of genes are responsible for the development of the various organs, these genes are transcription factors, i.e these genes regulate which genes are expressed and which are not. These genes and their interactions were nicely put into a model called the ABC model that explains this interaction, below is a rough diagram by me which explains this.

    When A genes are expressed alone, sepals form. When A and B genes are expressed together petals are formed, when B and C genes are expressed stamens (male reproductive organs are formed and when C genes alone are expressed carpel (female organs) are formed. If any of the genes is unavailable due to mutations then the flowers do not develop properly in the sense they have transformation of the organs. For eg, if the plant has a mutant B gene, then the flowers will only have sepals and carpels, but no petals and stamens.

    In orchids this matter is much more complicated due to their asymmetrical flowers with different type of petals. In orchids there are 3 different genes that regulate the B gene function in the three different petals. Below is a diagram which explains the same

    In the outer tepals only one B gene is expressed, in the inner tepals two B genes are expressed and in the lip three B genes are expressed as can be seen in the first picture of a normal Phal flower. Sometimes due to some mutation the third B gene which should be expressed in the lip crosses the boundry and also expresses in the the petals, this leads to the peloric nature of the petals in which the petals look like lip. The transformation can be in varying degree depending on the expression levels of this gene, as can be seen in the mutant peloric phal.
    Sometimes it can also happen that due to some mutation the third b gene is not functional and is not expressed, in such circumstances the lip is converted to a petal like organ, this is common in many Paphs and Phrags as is seen in the following picture.

    So basically it is a general interplay of genes. I hope I have cleared the air over the peloric orchids and the science behind it. The figures are from the research paper,
    "Why are orchid flowers so diverse? Reduction of evolutionary constraints by paralogues of class B floral homeotic genes" by Mariana Mondragón-Palomino and Günter Theißen from Jena, who are close research associates of our group. For anyone interested here is the link to the paper


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