Cycads are an ancient group of
Gymnosperm Plants thought to be among the closest surviving relatives of
Flowering Plants. They originated in the Palaeozoic, and were once one of the
most abundant plant groups on Earth, but were almost wiped out at the end of
the Mesozoic, with all Cycads alive today being the result of a post-Cretaceous
radiation. There are 331 known living species of Cycads, the majority of which
are listed on the International Union for the Conservation of Nature’s Red List of Threatened Species, with more than 75% of known species currently thought to
be threatened with extinction.
Cycads are threatened by habitat loss and
invasive species (particularly Insects), but are considered to be most at risk
due to over-collecting for horticulture. The distinctive evolutionary history
of the plants, combined with their attractive appearance and drought tolerance,
makes them highly sought after by plant collectors, however they are slow
growing and difficult to raise from seed, resulting in a high market value for
mature plants, which leads to many being taken from the wild. Four species of
Cycad are considered to be extinct in the wild, and it is not thought it will
be possible to reintroduce them to their former environments, as Cycads are
usually co-dependent on single species of pollinators, typically Weevils, with
neither species able to survive in the absence of the other.
In addition to other problems,
many Cycad populations are known to suffer from inbreeding depression, a
phenomenon which occurs when much of a natural population has been wiped out
and most of the surviving members are closely related. Inbreeding depression
can lead to health problems as harmful recessive genes become widespread, and
typically leads in reduced fertility. In extreme cases this can result in
populations with numerous living individuals, which can be observed and
recorded by naturalists, but which have no prospect of long-term survival.
In a paper published on the
International Journal of Plant Sciences in January 2015, a team of scientists
led by Patrick Griffith of the Montgomery Botanical Center in Florida discuss
the results of a genetic study of the Sinkhole Cycad, Zamia decumbens, both in the wild and at the Montgomery Botanical
Center, where a population of Sinkhole Cycads has been established from wild
collected seeds, in order to determine how the Florida population reflects the
genetic diversity of the wild population.
Specimens of the Sinkhole Cycad, Zamia
decumbens, growing wild in southern Belize. Griffith et al. (2015).
The Sinkhole Cycad was first
described in 2009, and has two main wild populations comprising 234 and 183
plants in two limestone sinkholes separated by 7 km in the Maya Mountains in
southern Belize, plus a few scattered populations on hilltops in the same area,
comprising no more than twelve individuals each. In addition there are 205
plants at the Montgomery Botanical Center, raised from seeds collected at both
sinkhole localities. The Sinkhole Cycad is considered to be Critically
Endangered under the terms of the International Union for the Conservation of
Nature’s Red List of Threatened Species.
The small size of the wild
populations meant that it was possible to genetically sample all of the wild
plants in the two sinkhole populations, as well as all of the plants at the Montgomery
Botanical Center. The plants were analysed for the total number of alleles
present in each population (an allele is a gene at a locus where more than one
variant can occur, for example humans have a single locus for eye colour, with
blue, brown and green eyes being produced by different alleles), in prder to
determine the genetic variation within each population, and determine how that
of the Montgomery population reflected total variation in the wild population.
The Montgomery population was
found to include 77.6% of the total wild genetic variation, including 70.0% of
the genetic variation at the first sinkhole and 73.6% of the variation at the
second. This is a little disappointing, as similar studies on wild and captive
Palms have found a much higher proportion of alleles in smaller captive
populations, although it is not altogether unexpected given the biology of
Cycads, as not all male plants produce pollen every year, so that seeds
collected from different female plants in a small population are more likely to
share the same father. It also suggests that the two wild sinkhole populations
are genetically distinct, and that plants collected from the two sites should
be curated separately.
Based upon this study, Griffith et al. make four recommendations for
future projects in which ex situ populations of Cycads (or other plants) are
being established to act as a genetic reserve for wild populations. Firstly the
collecting strategy should reflect the known biology of the plants. Secondly
where plants are collected from more than one population, these plants should
be curated separately. Thirdly multiple accessions (collections of seeds from
separate mother plants) should be collected and maintained. Finally seeds
should be collected on multiple years.
See also…
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