Developmental programs, alternative

Alternative developmental programs often provide flexibility to an organism... 

The genetic developmental program of an organism is encoded in the DNA. The expression of the program is implemented initially at the transcriptional level by a host of transcription factors that act at appropriate times and in appropriate places. Their action usually requires the presence of many preformed compounds. Alternative developmental programs may be used by unicellular organisms to adapt to new environmental conditions or by multicellular organisms to differentiate tissues.6... 

More complex alternative developmental programs are followed by colonial forms of bacteria such as the myxobacteria. The life cycle involves aggregation of cells and formation of fruiting bodies as well as sporulation.185... 

What is special about the testis that requires the absence of the circadian clock so pervasive in other tissues The testis has a number of characteristics, which make it quite different from other tissues. The testis contains spermatogenic cells that perform a constant and complex cell differentiation program where reductive cell divisions occur. It may be that the complex pattern of gene expression engendered by the circadian clock leads to unfavourable interactions with the developmental process of paramount importance to the testis. Alternatively, the normal oscillation of clock gene expression may be distorted by other transcriptional regulators or co-activators which are only present in seminiferous tubules (Sassone-Corsi 2002). 

The one millimeter long adult nematode Cae-norhabditis elegans contains only 959 somatic cells. Tire lineal descent of all of these has been traced.236-240 Tire development follows an almost exactly defined pathway with 113 programmed cell deaths during formation of the 558-cell newly hatched larva. In addition, each adult worm contains 302 neurons that make about 8000 synapses. This little nematode also has an alternative developmental pathway. The larvae shed their cuticles in four consecutive molts. If the food supply is inadequate, they enter a persistent nonfeeding state in which they may survive for months and are able to resume development when conditions are appropriate.241... 

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