The area of developmental genetics has also received major attention of geneticists in recent years to answer questions like the following. What are the relative roles of nucleus and cytoplasm in differentiation? How can mutations be used to probe into developmental processes? A unique example of the study of development is the embryonic development in Drosophila and sea urchin, where the initial cell divisions in the zygote are all similar and are controlled by the cytoplasm derived from the mother, but later, some cells divide slowly than the other, producing a pattern. How is it achieved? Is there a genetic control governing it? In plants also developmental mutants for flower development (e.g. stamens modified into petals) have been isolated and studied leading to the isolation of genes controlling development of floral organs in plants like Arabidopsis and Antirrhinum. The techniques for the study of developmental genetics are more difficult, but the area certainly rewarding and results much more exciting. For regulating the developmental process, temporal genes have been identified in several cases. These genes prepare a programme for the regulation of the expression of different genes in time and space leading to differentiation and pattern formation.
On the origin and evolution of the floral developmental genetic program. Relationships between angiosperm and gymnosperm subjects of current genetic studies are indicated in accordance with current phylogenetic trees, and hypothetical “protoangiosperm†stages are inserted below angiosperms to illustrate hypothesized steps in floral origin from gymnosperm cones (1, 2). Color schemes on the right depict hypothesized (1, 2) and established gene expression patterns of ABC genes (5–7, 11, 12) in the evolution of the floral regulatory program. Reduction of B function in the distal region of male gymnosperm cones results in female development (carpels) in a bisexual protoangiosperm I, and subsequent reduction of C function at the proximal region leads to the replacement of stamens by sterile perianth organs in protoangiosperm II. Expression data for basal angiosperms (11, 12) suggest that a “fading borders†mechanism of organ identity determination was active before the establishment of the strict expression domains of the ABC(E) model in Arabidopsis (13–15). For comparative purposes, protoangiosperm “flowers†are depicted with condensed axes, although this step may have occurred between stages I and II (2); A function is included despite its controversial status. Persea americana and Arabidopsis thaliana flowers are enlarged to compare their morphologies: both have whorled phyllotaxy, but in Persea an undifferentiated perianth of tepals surrounds the stamens and carpels, whereas a dimorphic perianth with green sepals and white petals surrounds the reproductive organs of Arabidopsis.
