The Evolution of Cacti from Pereskia

Evolution of the cacti from pereskia
Cacti are desert plants that grow in a leafless form. Their ancestors
pereskia composes seventeen species with regular development of roots.
Erika J. Edwards, Reto, Nyffeler, and Michael J. Donoghue conducted a
study on the evolution of the cacti. Their hypothesis sought to
establish a phylogenetic relationship based on the cactaceae. They
sought to study by inferring from the DNA sequence data. The data
derived from five gene areas that represent the three plant genomes. The
study involved the analysis of eight species of pereskia located around
the Caribbean basin. Other species in South America also served as
specimens. The study results centered on early gradual evolution of a
stem based photosynthesis and the evolution of stem stomata, the stem
cortex, and the evolution of specialized tissue system of photosynthesis
respectively. This paper seeks to review the study conducted by Erika J.
Edwards, Reto Nyffeler, and Michael J. Donoghue and further analyse it
from the Darwinian theory and concept of natural selection. Below is a
short summary of the methods and materials used in the study.
Materials and Methods
The researchers isolated genomic DNA from dry or fresh leaf or cortical
tissue. The procedure used a modified form of DNeasy Plant Mini kit. On
occasional instances, they experience mucilaginous extractions. This led
them to a repeat the precipitation process. Five different gene areas
representing the three plant genomes got used as the study specimens.
They got amplified and sequenced by means of standard primers. Polygenic
analyses got used to arrange the data collected into two groups
comprising of nuclear as well as mitochondrial/chloroplast. These groups
got analyzed in combination and separately.
The study applied MP analyses to perform heuristic searches by use of
starting tree. ML analyses aided in choosing molecular evolution models
by applying model-test 3.5b. The study formed hypotheses for testing the
alternative ancestral roots of the cacti: to establish that pereskia
clade formed the ancestral roots.
The evolution stem photosynthesis
The research established that there exist no direct reproductive traits
uniting the SSA pereskia clade and Andean with major lineages of cactus.
However, the study phylogenic hypothesis provided insight into the early
evolution of the stem. The stem evolved as the main photosynthetic
tissue of the cacti. There exist various characteristics postulated as
potentially vital in the process of evolution of the cacti to a leafless
condition. The study established that leaves develop on two major places
on the cacti. The leaves seem microscopic. The study showed that leaves
of all cacti emerge from the areole. In the same way, the leaves of
pereskia emerge directly from the areole. Unlike many other desert
plants, the cacti and the pereskia do not invest on the leaves before
paying attention to stem tissue. As a result of lack of these traits in
pereskia and cacti, it could be argued that there existed a transition
from leaf to leafless state in the cacti. However, the pereskia does not
offer good grounds for asserting such a conclusion.
The study further established some traits that link the Andean and SSA
pereskia clade with the major cacti. Further, the study showed members
of caulocatus clade grew stomata on their stem`s epidermis. They also
delay in their bark formation. This equates to the stem
photosynthesizing trait of the cacti. However, the maihuenia species of
cacti lacks this trait. All do not possess stomata in their stem. The
study postulated that the presence of the stomata on the stems of the
maihuenia species got lost along the lineage. The today’s maihuenia
composes of two species located in cold geographical areas of Patagonia
and south of Chile. The study did not clearly establish the cause of
lose in these traits. However, it deemed unlikely that the species
photosynthetic organ is the stem.
The study established that the members of pereskia clade in the north do
not possess stomata on their stem. They also do not delay in forming
their barks. The study did not ascertain the cause of loss of those
traits in the clade from the north or their gain in the cauloactas. It,
however, ascertained that the presence of stomata on the stem and delay
in bark formation still existed in the cactus before opuntioid and
cactiod diverged. The study also ascertained that the evolution of the
two species implies a correlation.
Pereskia`s stem transformed during cactus stem evolution. Hypodermal
layers containing thick walled collenchymatous cells developed on the
pereskia stem. Intercellular air spaces also developed. This was to
allow diffusion of carbon dioxide, which partakes, in the process of
photosynthesis. Cortical chlorenchyma tissue also arranged into a spongy
mesophyll on the pereskia stem. Palisade layers similar to those
possessed by photosynthetic leaves also formed. This major development
deemed to make an efficient process of photosynthesis for the evolving
cacti. The study showed that stem stomata evolution and delay in bark
formation came before the cortex modification. These modifications made
it possible for the stem to conduct photosynthesis.
Historical geographical origins of the cactus
The study established that the cacti are not as old as it may appear in
other researches including those of Baxbaum (1969) and Leuenberger
(1986). The study, however, confessed limited knowledge in ascertain the
geographical origin of the cactus. This deemed hard due to the cacti
ancestors’ limited knowledge. However, the study showed that there
existed a separation in the lineage of the cacti. There emerged from
this separation the primary southern and the primary northern clade.
While the northern clade comprises of scattered species in northern and
central South America, Hispaniola, and Cuba, the northern clade exists
in Argentina and Chile. The pereskia clade thrives in Benelivia and
Peru.
The Darwinian theory of natural selection and the evolution of cacti
Natural selection remain to date a famous theory by Charles Darwin
(Darwin, C., & Beer, G.1998). The main tenant of the theory deems that
evolution results from variation in generations of species and the
difference in survival of species. Those species that develop the traits
needed for survival get more chances of reproducing. The produced
offspring carries the genetic advantage of their parents. This
translates to the process of natural selection. Only the best suited
species survive. In order for natural selection to take place, there
exist a number of factors involved in causing evolution. Firstly,
species need to emerge and disappear, and as they exist, they undergo
change. Secondly the organisms must belong to a common ancestor. From
the common ancestor, the species diverge. Thirdly this divergence of
species takes the form of one species diverging to form another species,
and fourthly, this process takes a gradual development over a long time
(Mayr, E. 2001).
In study review above, the Darwinian theory and concept of natural
selection apply to the evolution of cacti. It appears evident from the
study that the seventeen species of the cacti shares a common ancestor:
the pereskia. This falls in line with one of the major tenets of
Darwin’s theory. From the common ancestor pereskia, the various
species of cacti emerged with different traits but with remarkable
commonality. They all lack leaves. Moreover, they all possess stomata on
their stems. According to Darwin theory, species undergo gradual change
over a long time. During this time, the species adapt to various
environmental or natural conditions. This, according to the study
reviewed, fits the cacti case. The stem of the pereskia underwent
gradual transformations to enable it evolve into a photosynthetic organ.
Thick walled collenchymatous cells developed on the pereskia stem.
Intercellular air spaces also developed in order to facilitate
photosynthesis. The stems also developed the stomata. These
transformations had to occur if the species pereskia needed to survive
to the next generation.
The study also showed that there existed a major divergence of the
species. It divided into the primary southern and primary southern
clade. Each of these grows in different geographical locations. The
maihuenia survives in cold dry areas of Pantagonia. It lacks the stem
stomata present in other species. However, it well adapts to its natural
environment.
In conclusion, the study reviewed agrees with the Darwinian theory of
natural selection in its major tenets
References
Darwin, C., & Beer, G. (1998). The origin of species. Oxford: Oxford
University Press.
Edwards, E.J, Nyffeller,R, and Donoghue, J. (2005). Basal cactus
phylogeny: Implications of
pereskia (cactaceae) paraphyly for the Transition to the Cactus Life
Form. American
Mayr, E. (2001). What evolution is. New York: Basic Books.
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