C. elegans Worm Laboratory Exercise
SDB Boot Camp, NMU, Albuquerque NM 8/4/10
While C. elegans embryos are great for observing and studying development; C. elegans adult
worms can also be used as an undergraduate laboratory exercise in genetics with a cross that
demonstrates the independent assortment of two unlinked traits, similar to Mendel’s pea plant
crosses, resulting in a 9:3:3:1 ratio of observable phenotypes. And, like Mendel’s peas, C.
elegans can be self-crossed or outcrossed (see more below). One of the many advantages of
C. elegans is that the experiment can be accomplished in about a week’s time, with three (and a
half) short laboratory sessions. Those sessions entail: 1. identifying and learning to handle
(“pick”) male and hermaphrodite worms, 2. setting up the cross, 3. picking a single F1 animal to
a fresh plate, and 4. scoring the phenotypes of the F2 progeny. I have used this exercise with
biology classes at Castle High School in Newburgh IN where my daughter, Jill Ohlsen, teaches
AP biology. I visit her class on a Thursday and Friday, for two 45 min classes, and in that time
each student is able to complete the cross. On Monday the students look at the results of their
crosses and individually plate one F1; by the following Thursday or Friday they score the
phenotypes of the F2 progeny. My daughter handles these last two steps. She also usually
introduces the class to the organism in an informal lecture before I visit. Some, but not all, of
the crosses work for the AP students (they are beginners!) and the class discusses the results
as a group. The concept of the independent assortment of two unlinked, recessive genes does
come across and most students can explain it after the laboratory work is complete.
Today we will condense these multiple sessions into one! Like Julia Child did with cooking
classes, I hopefully will be able to bring the F1 and F2 results with me for you to observe.
For this session you will receive:
•a plate of wildtype (N2 strain) worms to observe both males and hermaphrodites,
including adults and the L4 (fourth larval stage). With the tool provided, a “worm pick”,
you should first work at picking individual worms and placing them on a clean plate. It
takes a bit of practice to be gentle with the worms and with the agar; don’t gouge either.
•a plate of uncoordinated mutant worms. The unc-13 (e51 allele) (the strain available at
the Caenorhabitis Genetics Center (CGC) is MT7929). These worms have a mutation in
the unc-13 gene, which codes for a complex protein that is a conserved neurotransmitter
release regulator that affects syntaxin at the neuromuscular synapse and maps to
Chromosome I. The mobility of unc-13 worms is severely limited. Instead of moving as
an S, they are paralyzed and coil into a C shape. Both the unc-13 and dpy-17 mutations
were isolated by Jonathan Hodgkin, one of the first C. elegans investigators.
•a plate of dumpy mutant worms. The dpy-17 (e164) allele (the strain at the CGC is
CB164). These worms have a mutation in the dpy-13 cuticular collagen gene, on
Chromosome IV. dpy-17 worms have defects in their cuticle, which make them shorter
and fatter than wildtype worms. You will get a “mating” plate that was set up in the
Bennett laboratory last week with a single L4 stage (fourth larval stage worm that has
not yet started producing oocytes, so the worm has no self progeny) dpy-17
hermaphrodite and six dpy-17 males (more males up the chances the genetic cross will
occur). Hopefully there will be many dpy-17 males on your plate. Both unc and dpy
mutant phenotypes are easily scored; even the three-five year olds at my
granddaughters’ daycare center at the NSF in VA could distinguish these unc and dpy
worms from wildtype a few weeks ago when I gave a “worm show” there.
•Your task for today is to set up a cross by picking a single unc-13 L4 hermaphrodite and
gently placing this unc-13 worm with 5-6 dpy-17 males on a “mating plate” (on which the
E. coli lawn is confined to the center of the plate so the males will not crawl off and get
lost). After an unc-13 x dpy-17 mating, the F1s should be doubly heterozygous progeny,
all appearing wildtype (one can also tell if the mating did not work, as the progeny will
then all be unc, like their mother). I hope to have F2s to show that in the second
generation most of the worms will appear wildtype, but others will be dpy or unc, and a
