HISTOGENESIS AND GENETICS OF A PLASTID‐CONTROLLED CHLOROPHYLL VARIEGATION IN TOBACCO

A mosaic pattern of leaf variegation in a sector on a normal green tobacco seedling resulted from maturation of both green and white cells within the tissue derived from the second histogenic layer (L–II) of the shoot apex. While mature cells of L–II origin contained either green plastids or colorless plastids, single cells in young leaves contained both normal green plastids and colorless, defective mutant plastids. The genetic determiner of the defective plastid type, designated D P1 , was located in the plastid itself. Only a small number of mutant plastids were found in anyone cell. A threshold number of D P1 above which green plastids were inhibited and below which they developed normally is suggested. Random segregation of D P1 and d P1 in cytokinesis, close to the threshold value and without loss of either determiner, could account for the small, intermingled patches of green and white cells derived from the mosaic histogenic layer. The mosaic histogen usually remained mosaic; however, complete segregation during mitosis to D P1 or d P1 occasionally occurred in sectors within which lateral buds developed in a stabilized condition, either normal or mutant (green or white). Occasional replacement of L–III by L–II apparently introduced the mosaic condition into L–III of the original variegated plant and resulted in an additional pattern of variegation. Patterns of variegation in which the sporogenous tissue was derived from a stable L–II with homoplastidic cells produced only green or only white offspring. Variegated seedlings were obtained only when L–II was mosaic. Contrasting statements on the inheritance of variegations appearing in the literature result from failure to describe accurately the pattern of variegation. Description in terms of plastid types in mature histogens is essential, and plastid segregation in somatic tissue of mosaic L–II must be determined. Variegation should only be used as a general term and further described as stable when the histogens are homoplastidic or mosaic when the histogens are heteroplastidic. Complete description of the inheritance of a plastid mutant must record inclusion or loss in successive cell generations of sexual seedlings. D P1 is inherited only maternally and is found in fewer of the offspring than expected on the basis of its segregation in mitosis. When present in the egg cell and, therefore, the zygote, D P1 can be included in all 3 histogenic layers of the seedling, thereby producing many more variegated patterns than were possible from the original mosaic plant.