Worm Breeder's Gazette 7(2): 43

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Certain Heterochronic Mutations Alter the Temporal, Spatial and Sexual Specificities of Cell Fates

V. Ambros, B. Horvitz

Figure 1

Mutations in several genes result in heterochronic development (see 
previous Newsletters), i.e., cause certain cells to adopt fates 
normally specific for other cells generated at different times in 
development.  Heterochronic genes appear to participate in the 
specification or utilization of temporal information essential for the 
proper sequencing of developmental events.  We have recently observed 
that two heterochronic genes may also be involved in the control of 
spatial and sexual information.
The heterochronic mutant lin-28(n719) I displays precocious 
development of the lateral hypodermis: adult cuticle is formed at 
least one stage earlier than in wild type, and lateral hypodermal cell 
lineages skip the normal second stage ('S2') segment (Figs.  A and B). 
In addition to displaying these and other heterochronic defects, n719 
XX animals are partially transformed into males: V5.pa generates, in 
the L2, a normally male-specific ray, recognizable by lineage and 
morphology.  (In wild-type, V5.pa generates a postdeirid).  Thus, the 
developmental fate of V5.pa in n719 hermaphrodites is the same as that 
normally specific for V5.pppppa at a later stage of the same lineage 
in wild-type males (Fig.  C).
The mutant lin-14(n179) X at 25 C also displays precocious lateral 
hypodermal development (deletion of L1-specific cell divisions and 
precocious alae formation).  The double mutant lin-28(n719); 
79) displays enhanced precocious hypodermal cell 
lineages and alae formation (Fig.  D).  Furthermore, in the double 
mutant, any of the analogous blast cells V1-V5 can generate a lineage 
that apparently only V5 generates in the single mutant n719; e.g., in 
the animal represented in Fig.  D, V1.pa as well as V5.pa generated a 
male ray.  Thus, lin-28(n719); 79) results in a 
spatial (homoeotic) transformation in the fates of the lateral 
hypodermal blast cells.
These observations indicate that certain heterochronic genes can 
influence not only the time at which a developmental event will occur, 
but also at what position in the animal and in what sex it will occur. 
The products of these genes may be elements of a system by which 
cells are provided with and/or interpret three kinds of developmental 
information.  Alternatively, the spatial and sexual transformations of 
these mutants may be indirect results of their heterochronic 
development.  For example, if the critical developmental period for 
positional determination (e.g., V1, not V5) or sexual determination (e.
g., 'hermaphrodite' V5, not 'male' V5) is deleted, then this 
determination could not occur, and an alternative developmental fate 
may be expressed.
[See Figure 1]

Figure 1