Most ferns have very specific needs. Usually, they don’t stand plain light and have dramatic water envy. That’s
fair ferns, after all… Though some species can’t but stand apart from their next of kin spectrum. That’s for Blechnum chilense. Indeed, this Chilean fern is able to make it through drier and quite lightful places. In addition to the classics -that is, shady places under a fresh canopy, the plant is also growing in forest gaps.
A recent study* investigated selection under these very different life conditions. Natural selection studies in ecological and organismal contexts relies on estimates of fitness, or to put it simply, reproductive success. In plants, that’s more or less a somewhat easy thing to do: counting seeds gives you a quite good guess of the fitness (well, a good guess of the maternal fitness -for the paternal fitness, you can make it with more formal paternity analyses but it’s a way more costly in terms of bench work, chemistry and work time). Here in ferns, things are easier since the organism produces spores after a meiosis (which is halfway in sexual reproduction as we regard it, since gametes are produced later during the haploid generation, and gametes fusion, the gamy gamy, achieving sex, occurs later in basal plants like ferns). So estimating the gross number of spores or in this study of sporangia (the basic structure producing the spores) is a reasonnable guess of individual reproductive success… Don’t overlook the easiness of the task though. It’s conceptually simple. But reading the article, is something put that way:
Reproductive output was estimated from sporangia production. The number of sporangia per mm3 was counted in one pinna using a stereomicroscope (Nikon, Fukuoka, Japan), then multiplied by total number of pinna of the frond and total number of fertile fronds, thus estimating reproductive output of individuals.
Means a lot of tedious counts to work out. A lot, cause you have to do it for each single individual from your sample, and significant results often only come out a large enough sample size, especially for small effects. Ecological work requires a lot in sometimes rather time-consuming tasks.
Then, when you have both estimates of reproductive success and measured characteristics of your sample, the effect of natural selection is assessed in basically three steps. A first step are simple correlations of each single measured character and fitness. This is the “gross natural selection effect”. The predicted fitness that is associated with values of a certain phenotype. But at this point, no causation is necessarily to be assumed. Indeed, the actual selection may be exerted on another trait, and these two traits may be correlated so that you infer selection on the wrong characteristic. So there’s a risk that you ascribe a selective effect to a trait when there is actually none. Fortunately, the second step usually clear things up. That’s a multivariate analysis, it allows an improved estimation of the actual correlation between a trait and fitness taking into account any other correlative effect from your measures. Grossly, the simple regressions gives you an idea of both direct and indirect effect of selection on the trait, while the multiple regressions brings up an estimate of the actual effect of selection. (The third step is the inclusion of squared measurements and is aimed to detect possible optima within phenotypes, that is, when intermediate value have greater fitness).
So what is the consequence of growing in so different places? First, survival was assessed, and it appears that one trait was significantly associated and positively correlated with survival in forest understory: leaf size. The bigger your leaves, the better you survive from being a plantlet into a mature plant. In gaps, it is the reverse: the smallest your leaves, the greater odds that you survive ’til adulthood. But in gaps, there’s a second condition for survival: it’s also associated with a greater Water Use Efficiency (WUE, a physiological measure expressed as the ratio of photosynthetic capacity on stomatal conductance). Second, fecundity (as a proxy for reproductive success/fitness) is informing us that there’s selection for bigger leaf size in understorey, while the main selection criteria for reproducing in gaps was WUE. Drought is a rude guy!
Whereas smaller leaves lead to a reduction in leaf transpiration that is beneficial when growing in drier places, producing greater leaves is beneficial in shade because light is then more easily captured. Selection favoured characteristics related to economy of water in plants of the gaps, while in populations of forest understorey, traits increasing light capture were favorised. And these characteristics are more or less negatively correlated: bigger leaves may increase light capture in shaddy places, but they really aren’t efficient to prevent water loss through transpiration in full light places… Seems like you can’t have it both ways: either you’re good in moist forests, or in the drier gaps.
What is interesting here that even if there is some phenotypic plasticity, since produced leaves are (about two times) smaller in places with more light, there is also some genetic factors to be selected for (as revealed through clonal propagation experiments). That’s expected of course, because the different selection regimes each are maintaining their own favoured sets of genes. So that patchiness of micro-environments is sometimes helping you keeping diversity of some sort.
And there are some questions that would be worth asking… How frequently would such differentially adapted fern “strains” cross?
Do we have any idea what distance can gametophytic sperms swim? Or does spore dissemination do all the work to keep strains growing together in patches?
Finally, why did the fern cross the gap?
Creationist: “yes, but they are still ferns…”
* Saldana A., Lusk C. H., Gonzales W. L. and E. Gianoli (2007). Natural selection on ecophysiological traits of a fern species in a temperate rainforest. Evolutionary Ecology 21: 651–662.
Picture of Blechnum chilense courtesy of species_snob.