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Mistletoes - biogeography

Loranths of the intrusive element

How do we account for the remaining Australian loranths? These fall into two groups, but they appear to have a common geographic pattern. One group comprises the 12-chromosome genera Decaisnina click to view image, Amylotheca click to view image and Lysiana click to view image. Their 12-chromosome relatives are tropical genera with their centre of concentration in southeast Asia. Indeed most species of Decaisnina occur in Indonesia and the Philippines, and Amylotheca is also represented in Thailand and western Indonesia.

 

Genus
Chromosome
number
Number
of species
Distribution
Intrusive

Indogondwanan
(old Asian)

Macrosolen

12

30

India to New Guinea

Lepeostegeres

12

9

Malaya to New Guinea

Decaisnina

12

25

Philippines - Australia

Cyne

 

6

Philippines - N Guinea

Amylotheca

12

38

S Asia to Australia

(young endemics)

Lysiana

12

8

Australia

Benthamina

9

1

Australia


Benthamina alyxifolia
Benthamina alyxifolia

The other group comprises the genera with 9 chromosomes, somewhat smaller than those of the Amyema species group. Dendrophthoe click to view image is also an Asian genus, with a few species reaching Australia or endemic there. Benthamina, although a local endemic in eastern Australia, is clearly related to other Asian genera in the Dendrophthoe alliance.

It therefore appears that these mistletoe genera in the Australian region have had a different history from the original Gondwanan stocks. They are more recent arrivals, having crossed Wallace’s Line from the northwest after contact between the Australian and Indonesian plates. They have diversified to some extent in their new homeland, but are still predominantly species of tropical or monsoon habitats. Lysiana is the most specialized of the 12-chromosome loranths; it is presumably a young genus which has differentiated in Australia, but even in Lysiana most of its species are tropical. Benthamina is apparently an outlier of a group of genera which otherwise now only reaches the Lesser Sunda Islands.

Given the apparent Gondwanan origin of Loranthaceae, how do we explain the intrusion of loranths into Australia from the north? The likely explanation involves the geophysical history of India and parts of southeast Asia as Gondwanan fragments. During the time India was rafting northwards towards collision with Asia in the middle Eocene, its climates may have been continuously equable. India may therefore have carried a differentiated flora of Gondwanan origin to Asia at the same time as another subset was differentiating on the Australian fragment (below, numbers indicate chromosome number). This Indian/Asian subset of Loranthaceae appears to be the source of all African loranths, as well as the intrusives which have reached Australia, there meeting up with Gondwanan relatives they sailed away from tens of millions of years earlier.

click to enlarge map click to enlarge map
click to enlarge

 



Written by Bryan Barlow, updated 1 December, 2011 by webmaster, ANBG (anbg-info@anbg.gov.au)