Caladium steudneriifolium is
a plant found in the rainforest of south east Ecuador, that has evolved a
method to avoid herbivore attacks. The plant has evolved two distinct leaf
phenotypes; one which is green, and one which is variegated. The development of
the variegation phenotype is believed to be (Soltau et al., 2008) a form of mimicry; posing as a leaf that
has been attacked by the larvae of the mining moth. This mimicry was seen in one
third of C. steudneriifolium leaves randomly chosen and studied by
Soltau et al. (2008), and it was estimated that infestation by a mining
moth was 4 – 12 times higher for the green leaf phenotype than it was for the
variegated form.
To support their hypothesis of
damaged leaf mimicry, Soltau et al. (2008) set up a test to decipher if
the mining moth was using visual cues to detect and avoid previously hosted
leaves. To do this they painted green leaves with white correction fluid in a
similar pattern to the variegated leaves. To ensure that neither the chemistry or
texture of the correction fluid was effecting a female’s propensity to lay
her eggs, un-pigmented correction fluid was placed on green leaves. It was
then established that there was no significant difference between untreated and
treated leaves. It was found that the leaves painted with white correction
fluid experienced the similar low rates of moth attack as the leaves with natural
variegation; herbivore attack mimicry was then successfully supported as a
hypothesis and the female moths are indeed using visual cues to determine
oviposition sites.
 | ||||||
| a. is a plain leaf, b. is a leaf infected with mining moth larvae, c. is a variegated leaf, d. is a plain leaf painted with correction fluid. Adapted from Soltau et al. 2008. |
Although variegation successfully
reduces the risk of mining moth oviposition, the overall growth and health of
the leaf is compromised. This is because the leaf sacrifices chloroplasts to
create the whitish patterns; which effects the absorption and utilisation of
light, hence reducing overall net photosynthesis.
Despite the leaf’s handicap of a
lower net photosynthetic rate, variegated leaves become comprehensible when the
life span of a damaged leaf is taken into consideration. Once epidermal damage
is caused by the mining moth larvae, the leaf is then open to attack from
varieties of fungi which can quickly destroy leaves. Leaf variegation in Caladium
steudneriifolium has consequently been shown to be of high selective
advantage, regardless of the loss of photosynthetically active leaf area.
Very interesting! This is a fascinating look at the trade-off between lowered photosynthetic ability and decreased predation risk. Is this phenomenon specific to this group of plants, or is it more wide-spread across the plant kingdom?
ReplyDeleteVariegated leaves are very common across angiosperms; mostly understory species in temperate and tropical climates (Soltau et al., 2008). Because there is such a stable co-occurrence of the different leaf colour morphologies, it implies that there must be selective pressures to support the variegation. There are many hypotheses as to why variegation exists, and different explanations can apply to different species. For example Byttneria aculeata was studied by Smith (1986) who discovered that the variegation on this species correlated with a reduced canopy and enhanced sunlight exposure.
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