I believe that part of making great discoveries is
picking the right subject to use in your investigation. To this end I have been
working to become familiar with many of the odd reproductive strategies and
unusual genetic systems within Coleoptera. At the top of a list like that has to
come Micromalthus debilis LeConte 1878.
This apparently ancient member of the suborder Archostemata has such a
complicated and bizarre life history that I felt I should write it up just to
make sure that I understand it personally.
First a few definitions to lay
the groundwork.
Paedogenetic: Producing young while in the
immature or larval state.
Thelytokous parthenogenesis: Producing only
females from unfertilized eggs.
Arrhenotokous
parthenogenesis: Producing only males from unfertilized eggs.
Amphiterotokous
parthenogenesis: (I can't
actually find a definition for this word... but it is used...) to describe larva
that appear as though they should have produced a male but for some reason have
failed to and after a delay begin producing (parthenogenetically)
females.
Haploid and Diploid: A
diploid organism has two copies of each chromosome while a haploid organism has
only one copy of each chromosome.
Ovoviviparous: Refers to animals that produce
eggs but retain them inside the female body until hatching occurs, so that
"live" young are born. Some of the source papers for this article refer to these
beetles reproducing viviparously but I can find no evidence of this in anywhere
so I am using this more conservative term.
I created the flowchart
below to show all of the different possible developmental pathways. It is
amazing to think of the amount of morphological data stored in this genome.
These 10 chromosomes are capable of producing so many unique phenotypes.
Learning what signals are important in determining the appearance of these
phenotypes would be quite interesting.
I found the easiest way to think about this is to start with the
triungulin larva. This larva always develops into a cerambycoid larva. From here
we can go the simple path of producing pupa and then an adult female, or the
alternate option to molt and become a reproductive larva. There are three type
of reproductive larva the most common is the thelytokous larva which is
responsible for most reproduction in a population. This larva Ovoviviparously
gives birth to large numbers (up to 20) triungulin larva. The other two options
are to produce a single male curculionid larva that consumes the mother before
pupating into the adult male form or if the male does not develop for some
reason the larva can after a delay begin producing triungulin larva though not
usually in the numbers seen in the thelytokous larva form.
Allan Charles
Scott published a paper in 1938 titled "Paedogenesis in the Coleoptera" were he
describes his discovery that the fate of larva are decided very early on. He was
able to discern significant differences in the ovaries of cerambycoid larva
allowing him to identify those that would pupate into adult females, thelytokous
larva or one of the other two classes of reproductive larva. These last two
classes of larva can not be told apart until much later and it may be that
arrhenotokous larva can develop into amphiterotokous larva under the right
conditions. The traits that Scott looked at were the number of lobes in the
developing ovary and the size of the egg cells.
On the left we see one ovary of the thelytokous larva, in the
middle we have the adult female ovaries, and on the right we have the ovaries
of the Arrhenotokous larva. (Figure from Scott,
1938)
Scott's 1938 paper also includes a drawing of all 5 reproductive
forms of Micromalthus debilis which I have reproduced and explained
below.
Adult Reproductive
Forms
D - Adult Females: Are diploid and have 20 chromosomes. They can
lay eggs that are presumably fertilized by males. These eggs are believed to
develop into triungulin larva capable of any of the various developmental paths.
Which development path is followed is most likely controlled by the
environmental conditions. Whether these females are capable of laying
unfertilized eggs that will develop is unclear from the research that I have
found. It is also interesting to note that Scott who has done the most thorough
examinations of this species found never more than two eggs close to maturity in
females (in contrast the Thelytokous larva can produce broods of up 20 larva).
This is particularly odd if we assume that this stage is the primary dispersal
stage.
G - Adult Males: Are haploid organisms and have only 10 chromosomes.
These males can presumably fertilize adult females they do definitely produce
sperm though meiosis does not progress in the normal fashion (only 2 spermatozoa
are produced from each primary spermatocyte.
Paedogenetic Forms Reproducing
Parthenogenetically
B - Thelytokous Paedogenetic Female: This form
of the beetle is the most prolific. Via parthenogenesis and ovoviviparity it
produces broods of the triungulin larvae. Like other triungulin larva in this
species they are capable of following any of the developmental
pathways.
E - Arrhenotokous Paedogenetic Female: This form matures a number
of eggs but only one of these is shed. This egg adheres to the mother after
being shed and hatches after a very short time. When it hatches the male
curculionoid larva begins devouring the mother immediately. This is the only
source for larva that will develop into the haploid
male.
C - Amphiterotokous Paedogenetic Female: this form has proven to be
essentially a male producer (Arrhenotokous Paedogenetic Female) in which the
development of the male is arrested and female-producing eggs develop
secondarily.
Other stages
A - Cerambycoid Larva
F - Pupa
If there are any
obvious or not so obvious mistakes please feel free to let me
know.
Source Papers
Pollock, D. and Normark, B. (2002). The life
cycle of Micromalthus debilis)(Coleoptera: Archostemata: Micromalthidae):
historical review and evolutionary perspective. Journal of Zoological
Systematics & Evolutionary Research 40, 105-112.
Scott, A.
(1938). Paedogenesis in the Coleoptera. Zoomorphology 33, 633-653.