COLOR CODE FOR unc85-4

July 9, 2012

DHH interpretation

This mutant adult hermaphrodite (unc-85 allele e1414) shows defects in movement and egg-laying, with defects in postembryonic cell divisions, including the P cell lineages. This specimen was reconstructed in the ventral nerve cord to follow the possible cell fates among P cell daughters where the mutation has caused an incomplete lineage. The P cell daughters have still migrated into the ventral nerve cord, and have tried to adopt neuronal cell fates “appropriate” for their lineages (see White, Horvitz and Sulston (1982) Nature 297: 584-587.)

The mutation was originally picked by Bob Horvitz as isolate “Il10”, and this was one of about five unc-85 animals that were prepared for TEM (specimens were originally called Il10-1 thru Il10-5). Thin sectioning and electron microscopy was conducted by Nichol Thomson at MRC, and cells were marked on prints by Marilyn Anness.

The effect of the gene knockout was to block late cell divisions, sometimes during or just after DNA replication, so that the resulting cell was sometimes polyploid, with an enlarged nucleus or possibly even having a fused nucleus after an incomplete mitosis. The affected cell then proceeded to migrate and mature, adopting a neuronal fate, often including excess fine branches coming off the cell body, but with a synaptic pattern that matched one of the expected two daughters of the affected cell, but not the other. So for instance, when the precursor of VA/VB was blocked in division, the resulting cell might adopt an apparent VB fate, or the precursor of AS/VD might adopt a fate appropriate for VD. Other nearby neurons in the ventral cord undergo normal cell divisions, migrations and appear roughly normal in shape and connectivity. However, quite a few neurons show changes in cytology, including large membrane whorls in the somata, and admission of larger clusters of ribosomes into the neuron processes compared to wild type neurons.

More recently, unc-85 has been demonstrated to encode the homologue of the yeast Afs1 histone chaperone gene involved in gene silencing (see Grigsby and Finger (2008) Dev. Biol 319:100).

Motor Neurons (fates taken from MRC assignments in Blue file W and from Table 1 in White, Horvitz and Sulston, 1982)

1 P3.111 (VA/VB) [+ VC fusion?] excessively long axon ~VB fate

2 DD

3 (VD/AS) -> VD fate

4 DA

5 AS

6 P4.111 (VA/VB) [+ VC fusion?] very branchy ~VB fate

7 DB

8 VD

9 P5.111 (VA/VB), very branchy ~VB fate

10 DA

11 DD

12 VC

13 AS

14 HYP

15 P6.111 (VA/VB) poor axon outgrowth ~VB fate?

16 DB

17 P6.112 (expected to yield a cell death) ~ VC fate?

Interneurons

1 AVAL

2 AVBL

3 AVAR

4 AVBR