Carnivorous Plants
_______
III. Snap-Trap
2017-03-15
Updated 2018-6-10
1. SNAP-TRAP EVOLUTION
There are two snap-trap carnivores today: the Venus flytrap (Dionaea muscipula) and the waterwheel plant ( Aldrovanda vesiculosa). Overwhelming evidence from molecular phylogenetic reconstructions using different regions of DNA sequence indicates that Dionaea and Aldrovanda are sister to each other, strongly suggesting that these extant snap-traps evolved from a common ancestor.
The molecular evidence further suggests that Aldrovanda and Dionaea form a clade that is sister to Drosera (sundews). This implies that the common ancestor of Aldrovanda and Dionaea came from an ancient sundew-like plant (Subgerera Regiae + Arcturia)..
Snap-Trap Evolved Only Once
This most likely
means that the snap-trap mechanism evolved only once in the common ancestor of Aldrovanda and Dionaea. Therefore,
these snap-traps are very similar --- the basic
mechanism behind their snap-trap movement must have its origin in their
common ancestor.
That is not to say that these two snap-trap mechanisms we see today in the Venus flytrap and the waterwheel plants are identical. Of course, there are some differences. After all, both lineages had 30-40 million years of completely independent evolutionary paths. Not to forget also that one is terrestrial, the other is aquatic, both traps well fine-tuned in their respective, entirely different environments. We can surely find differences between these snap-traps.
We do well seeking similarities --- rather than dissimilarities --- when we
strive to unlock the mystery and wonder of these "most wonderful plants in the
world."
2. WHICH EVOLVED FIRST?
An intriguing question remains as to which lineage evolved first --- Aldrovanda or Dionaea --- from an ancient sundew-like plant? To put it another way, was the common snap-trap ancestor an Aldrovanda-like aquatic plant or Dionaea-like terrestrial plant?
My take --- Aldrovanda...
When we talk about which came first,
we are not suggesting that today's Venus flytrap gave birth to today's
waterwheel plant, or vice versa. We are discussing the possible ways in
which the ancestor of these extant plants diverged tens of millions
of years ago...
THE THEORY OF RECAPITULATION - "Ontogeny Recapitulates Phylogeny"
1) Lloyd (p.182) - In Venus flytrap, the trap tilts to the right - mainly in younger plants, just like the clear bend seen in the mature Aldrovanda traps (see Lloyd/Darwin) --- Regarding this trap posture, Lloyd commented on the distinct advantage for Aldrovanda but no benefit for Dionaea. However, Lloyd did not extend this observation he made to an evolutionary context. This tilting trait was acquired in the ancestor of Aldrovanda and was carried over to the Venus flytrap, even if it is no use for the Venus flytrap. It is often manifested in the younger Venus flytraps --- "Recapitulation."
2) Lloyd
(p.181) comments on a young Venus flytrap (2 mm trap) ----> "The number of
parenchyma cells ranges between two to four courses with large
interspaces, in this feature again resembling the mature leaves of Aldrovanda much more than do the thicker mature leaves of
Dionaea."
The structure of
the mesophyll of the Venus flytrap leaves is more like that of aquatic
plants....
3) Venus flytrap is very tolerant of flooding condition - possibly implying an Aldrovanda-like, aquatic ancestor. It is known that the Venus flytraps grow underwater for a few months without any harm. The Venus flytraps catch prey in the water, no sweat. Hurricanes and violent storms go through North Carolina every year, flooding the region where Dionaea grows. I assume it's been like this for the past 50 million years.
4) Aldrovanda seems to share more common reproductive features with Drosera than does Dionaea.
Comparison of
flower parts:
Flower / sepals / petals
Dionaea
pentamerous (=5), actinomorphic. flowers protandric (open
for 3 days)
Aldrovanda
pentamerous (=5), sometimes 4, actinomorphic obligately autogamous or self-fertilising.
(open for 2-3 hrs)
Drosera pentamerous
(=5), actinomorphic (generall)
tetramerous (=4) (Sect Lasiocephalla/1 species + Sect Bryastrum/1
species)
8-12-merous (Sect. Ergaleium/1 species)
Stamens -
anthers / filament
Dionaea
10-15 stamens
Aldrovanda 5
stamens (yellow anthers on white filaments)
Drosera
5 stamens with bithecate anthers ( only a few exceptions)
Pistil - Style
/ Stigma
Dionaea
single central pistil made up of 5 fused carpels -
fused styles - separate/undivided styles united style
Aldrovanda
pistil made up of 5 fused carpels. 5 separate/undivided styles each terminates in a white stigma
Drosera
pistil made up of 3-5 (more often 3) fused carpels - separate/undivided styles
(In Drosera, styles are major key character)
Ovary
Dionaea
superior, unilocular, spherical ovary. basal placentation Superior ovary -
single chamber
Aldrovanda
single spherical ovary, superior ovary / has 5 parietal placentas each bearing 8-13 ovules
Drosera parietal placentation
The ovary has five parietal placentas ...
Pollen
Dionaea
pollen tetrads are tetrahedral, pollen grains are operculate
Aldrovanda pollen
tetrads are tetrahedral, 3
pores with an operculum united in tetrads with 4 grains
Drosera multiple operculate pores (early-branching
subgenera Regiae + Arcturia)
inoperculate pollen (more derived subgenera Ergaleium +
Drosera)
Seed
Dionaea 8-20
seeds per flower. (1 mm, black, pear-shaped)
Aldrovanda
5 x (8-13) seeds per flower.
Drosera numerous
seeds per flower.
Glands
Dionaea stalked
and sessile glands
Aldrovanda stalked
and sessile glands
Drosera only
stalked glands (early-branching subgenera Regiae +
Arcturia)
stalked glands and sessile glands (more derived subgenera Ergaleium
+ Drosera)
Ref:
Bisexual Flowers: A
flower that has both stamens (male) and pistils (female) in the same
flower.
Unisexual Flowers: A
flower that has only stamens (male) or pistils (female) but not both.
Ref:
Monoecious Plants: Both
the male and female organs exist on the same
plant.
Dioecious Plants: A
male plant produces only male flowers and a female plant produces only
female flowers..
A
flower
is said to be
unisexual
if it possesses either stamens (male) or carpels (female) but not both.
A
plant
is said to be
dioecious
(unisexual) if it possesses only male flowers or female flowers (not
both). Or a
plant
may be
monoecious
with both male and female reproductive organs on the same plant, either
borne in the same flower (bisexual flower) or in different unisexual
flowers.
There are two types of self-pollination: (not to be confused with self-compatibility in plants (the ability to form seed using self-pollen).
in
autogamy, pollen is transferred to the stigma of
the same flower.
in
geitonogamy, pollen is transferred from the
anther of one flower to the stigma of another flower on the same
flowering plant. The pollen transfer uses a pollen
vector (pollinator or wind). Wind pollination is a quite common source
of self-fertilized seeds in self-compatible species.
Some plants have mechanisms to ensure autogamy:
such as flowers that do not open (cleistogamy),
or
stamens that move to come into contact with the stigma.
If a plant is self-incompatible, geitogamy can reduce seed production.
Although geitonogamy is functionally cross-pollination involving a pollinating agent, genetically it is similar to autogamy since the pollen grains come from the same plant.
Monoecious plants like maize show geitonogamy. Geitonogamy is not possible for strictly dioecious plants.
Ref:
Basal
placentation: placenta is at the base (bottom) of the ovary. Simple
or compound carpel.
Apical placentation: placenta is at the apex (top) of the ovary.
Simple or compound carpel.
Parietal placentation: placentas are in the ovary wall within a
non-sectioned ovary. Compound carpel.
Axile placentation: ovary is sectioned by radial spokes with
placentas in separate locules.
Compound carpel.
Free or central placentation: placentas are in a central column
within a non-sectioned ovary. Compound carpel.
Marginal placentation: only one elongated placenta on one side of
the ovary, as ovules are attached at the fusion line of the carpel's
margins . This is conspicuous in legumes. Simple carpel.
3. DROSERA--ALDROVANDA--DIONAEA
Molecular
evidence points to a common origin of the two snap-traps (Aldrovanda &
Dionaea), suggesting that the snap-trap mechanism evolved only once -- therefore, the basic
mechanism for these snap-traps must be similar... actually identical...
It is widely accepted that, in the case of Dionaea, the snapping
of a trap leaf involves buckling. This is due to the
initial convex curvature (doubly-curved) of the open trap lobes of a mature
specimen. This "snap-through" buckling (or "flipping") does increase the speed of trap
closure, a little. However, it has to be understood that the buckling,
if it does happen, is not the main, driving force of the swift leaf
closure, but rather, a result of it. The main cause of leaf closure is
the pressure differential created between the opposite sides (upper &
lower) of the trap lobes..... The same mechanism is responsible for the
swift snap-trap of Aldrovanda (no buckling here, though).
Molecular evidence
further indicates that the common
ancestor of these snap-traps diverged from an ancient
sundew-like plant --- perhaps the ancestor of the basal taxa, such as Drosera regia.
This implies the
basic mechanism responsible for the snap-trap operation has its root in
the tentacle movement (and leaf folding) seen in the majority of extant
sundew species (and in D. regia).
The basic
mechanism for leaf motion common throughout Drosera-Aldrovanda-Dionaea
evolution is most likely to be a sudden (or relatively quick) drop of turgor pressure on one side of the structure in question, creating an
imbalance of pressure on the structure to cause it to bend.... In this
process, the other side (epidermis) might be forced to stretch a little .... The
recovery of the bending (or snapping for that matter) is achieved as a result of the side (epidermis) that lost turgor pressure restoring
its lost pressure and then some
to counter the stretch of the other side. This is accomplished by slow,
normal, actual
growth.
THE SHIFT OF PRESSURE DIFFERENTIAL DURING TENTACLE
BENDING & SNAP-TRAP CLOSURE (2017-11-24)
Physical motions in plants are caused by different pressures between the opposite sides of the structure in question: one side expands, the other side shrinks, or both --- due to either turgor change or cell growth.
1) In Drosera tentacle bending, the pressure differential between the opposite sides of the tentacle (stalk) occurs near the base first, and then gradually moves upward in the direction toward the tentacle tip.
2) In Aldrovanda snapping also, the sudden pressure differential between the inner and outer epidermis in the motor region occurs near the midrib of the trap, and then gradually moves upward (within the motor region) during the tightening phase of the trap closure...
3) In Dionaea snapping, the sudden pressure differential on the upper and lower epidermis of the lobes seems to occur in the upper half of the lobes --- that effectively causes the snap-through buckling of the trap lobes --- and then the pressure slowly moves downward toward the midrib during the narrowing phase of the closure...
1. Drosera (sundews) ---- Illustrations
Trap type:
Adhesive - Stalked gland secretes mucilage, exhibits nastic/tropistic
movement to secure prey.
Sensory
cells: epidermal cells at the tip of the tentacle (covered by two layers of
glandular cells)
Perception of stimuli: 2 action potentials within 1 minute to cause a tentacle
bending...
Mechanism: Sudden drop of turgor pressure on one side of the
tentacle...
Tentacle
bending (nastic / tropistic) - leaf folding
ILLUSTRATION ========== DROSERA
2. Aldrovanda (waterwheel plant) ---- Illustrations
Trap type:
Snap-trap (aquatic).
Sensory cells:
trigger hair...
Perception of stimuli: 1 action potential to snap...
Mechanism: Sudden drop of turgor pressure on the inner epidermal cells in the
motor region...
motor region
- narrowing (free-side lobe / bristle-side lobe)
ILLUSTRATION ========== ALDROVANDA
In the
illustration below, I chose to show the frontal views of the trap (all views
except one lateral view) as if the leaf blade is extending toward you (not from
the tip of the trap, as in the video). You can see that by noting the bristles
shown on the left of the trap.
Aldrovanda vesiculosa snap-trap closure
Well, based on my repeated
trials of measuring the screen image of this video, I concluded that, indeed,
the A-B distance got shorter (in the left view of the video) after trap
snapping, and therefore, the Aldrovanda closure is driven by warping of
the motor zone of the lobes. This is in concert with the traditionally-held
view by many past investigators, including ...
3. Dionaea (Venus flytrap) ---- Illustrations
Trap type:
Snap-trap.
Sensory cells:
embedded in the indentation at the base of the trigger hair
Perception of stimuli: 2
action potentials within 20 seconds to snap...
Mechanism: Sudden expansion of mesophyll and loosening of the outer (abaxial)
epidermal cell walls...
ILLUSTRATION ========== DIONAEA
Copyright (c) 2017 Makoto Honda. All Rights Reserved.