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inappropriate elevation could delay vulval cell divisions during recovery
from L1 arrest. The influence of nutritional status on
mir-71
,
hbl-1
, and
lin-42
expression in the vulval lineages warrants further investigation.
12. A HETEROCHRONIC PATHWAY FOR LARGER
ANIMALS?
A question from the very beginning of the heterochronic gene studies
has been: “Is this pathway peculiar to nematodes, or does it recur in other
animals?” Of the core pathway components, a few have clear orthologs in
animals, including mammals. Others have homologs, but it is too early to say
whether those are likely to be performing similar roles.
Mammalian LIN28 has an established role in pluripotency, proliferation,
development, and cancer (
Shyh-Chang & Daley, 2013
). It is expressed in a
variety of developing tissues, consistent with a role in timing development
on a tissue-by-tissue basis (
Yang & Moss, 2003
). Its role at the cellular level
was explored in neural development where it was shown to have a function
remarkably like that in
C. elegans
(
Balzer, Heine, Jiang, Lee, & Moss, 2010
).
Continuous expression of LIN28 caused a kind of retarded development
where early fates (neurons) were reiterated at the expense of later fates (glia).
It remains to be seen whether all of mammalian LIN28's functions can be
traced to defects like those in
C. elegans
, or whether it has diversified its role
over evolutionary time.
LIN41 has also been shown to control development in the central ner-
vous system, and mouse LIN41 knockouts display a kind of precocious
development in this tissue (
Chen, Lai, & Niswander, 2012
). LIN41 is also
widely expressed and in many of the same tissues as LIN28 (
Kanamoto,
Terada, Yoshikawa, & Furukawa, 2006; Schulman, Esquela-Kerscher, &
Slack, 2005; Yang &Moss, 2003
). It is intriguing that these two genes might
represent a bit of the heterochronic pathway that has been conserved in
mammals. A great deal of work remains to be done to see whether they work
together to regulate developmental transitions.
13. CONCLUSIONS AND FUTURE DIRECTIONS
In the interval since Ambros and Horvitz first realized that a few
C. elegans
mutants could be classified as displaying developmental timing
defects, much progress has been made toward understanding how temporal
