Polyploidy: Would Sir Joseph Banks Approve?

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