If Sir Joseph Banks were to visit a suburban nursery
today, would he appreciate the wide range of hybrid plants that have been
derived from those he originally collected?
 |
| Fig. 1 - Sir Joseph Banks (National Portrait Gallery, London) |
Sir
Joseph Banks (figure 1) was a wealthy naturalist whose many exploits included
the exploration of the East Coast of Australia on the Endeavour in 1770. Some
of the original species collected by Banks and botanist Daniel Solander
included Banksia serrata, Castanospermum australe, Correa
reflexia, Callistemon citrinus, various Melaleuca species, Grevillea
mucronulata and the orchid Dendrobium canaliculatum ( Banks And
Solander Species List). Today, some of these have become almost unrecognisable,
due to continued hybridisation with related species over the years.
Why Hybridise?
Plant
breeders have long recognised the improved features in various hybrid plants;
these include larger flowers, fungal and pest resistance, longer flowering
season, larger fruits and increased rigour. Many hybrid Australian plants are
the result of crossing related species to produce varieties with the best
qualities of each.
 |
| Fig.2 - Banksia 'Giant Candles' (Royal Botanic gardens, Melbourne) |
For
instance, a wide range of Grevillea hybrids have been developed to incorporate
features such as prostate habit, vivid colouration and larger flower heads. The
hybrid Grevillea ‘Emma Charlotte’, the result of a cross between Grevillea
rosmarinifolia and Grevillea lanigera, is an illustration of this . Banksia ‘Giant Candles’ (figure 2) is another native hybrid – in
this case, between Banksia ericifolia and Banksia spinulosa. This
particular banksia has been deliberately selected for its larger than normal
flower spikes and its showy orange-red colour.
Perhaps
one of the best examples of hybridisation can be seen in the latest Kangaroo
Paw (Anigozanthus) varieties. Hybrids of the more pedestrian Anigozanthus
flavidus and Anigozanthus rufus, for instance, tend to combine both
hardiness and showy colours in a smaller, bushier form that appeals to home
gardeners.
Polyploidy
Gives Hybrids Their Edge
One weakness of many Kangaroo Paw hybrids, however, is
that they are sterile and consequently can only be propagated using tissue
culture or other asexual methods. This also applies to countless other hybrid
plants, such as commercial bananas, and is due to their polyploid genetic
status. A polyploid organism is one that has 3 or more sets of chromosomes
instead of the normal two. If the number of chromosome sets is uneven,
production of gametes, and therefore sexual reproduction, is impossible.
Although it can arise from plant hybridisation,
polyploidy can also be induced artificially using the alkaloid colchicine.
Treatment with this chemical has the effect of preventing cell plate formation
during cell division. As a result, chromosome pairs cannot separate and the
treated cells become ‘tetraploid’; that is, they now have four sets of
chromosomes instead of two. Such tetraploids, along with all polyploid species
possessing even numbers of chromosomes, are, however, capable of reproducing
sexually. Colchicine-induced polyploidy has produced some spectacular orchid
and African violet varieties, as well as more productive wheat, maize and other
crop plants.
Banks’
Probable Verdict
So what would Banks have thought about polyploid
hybrids? Although he was unaware of modern genetics and would not have
recognised the concept of polyploidy per se, Banks would have been aware of the
prevalence of hybridisation in plant and animal breeding experiments. He had
himself sponsored the improvement of British wool by introducing Spanish sheep
into the country, and was also eager to introduce economic plant species such
as breadfruit to new colonies. In the light of this, Banks may well have
encouraged today’s artificial selection techniques if he had known of them.
Moreover, he may not have been surprised to learn that
polyploidy occurs frequently in nature – George Caley, who collected plants for
Banks between 1800-1810 , was in fact one of the first to identify naturally
occurring hybrid varieties of Eucalypts. Indeed, around 30-70% of flowering
plants are polyploid as a result of paired chromosomes originally failing to
separate during meiosis, or because of the combination of distinct chromosome
sets of two different species to create ‘alloploid’ varieties.
According to Fawcett, Maere and Van de
Peer , these polyploid plants were probably instrumental in the
evolution of higher plant species and continue to play an important role today.
Banks, arguably one of the world’s greatest botanists, would no doubt agree
with this view.
References
Botanic Gardens Trust, "Banks & Solander
Species List", Department of Environment, Climate Change and Water NSW,
nswgov.au, accessed 24/01/2010
Australian National Maritime Museum, "Joseph Banks
and the Flora of the Australian East Coast", anmm.gov.au, accessed
25/01/2010
Fawcett, Maere and Van de Peer , 2009, "Plants with
double genomes might have had a better chance to survive the Cretaceous–Tertiary
extinction event", National Academy of Sciences, pnas.org, accessed
25/01/2010
Australian Native Plants Society, 2009, "The
Kangaroo Paw Family – Cultivation", asgap.org.au, accessed 25/01/2010
