(see Graphic)

Fungi are closer genetically to animals than to plants, say researchers who

have compared sequences of ribosomal RNA. Their work refines the genetic

‘tree of life’ pioneered by Carl Woese of the University of Illinois. He

divided living things into three ‘domains’ – eubacteria, archaebacteria

and eukaryotes (New Scientist, Science, 11 August 1990).

Traditionally, living things are classified into five kingdoms on the basis

of their appearance, not on the basis of their genes. Advocates of the

genetic approach, however, believe it shows evolutionary patterns more

accurately; but others are unconvinced.

Mitchell Sogin and his colleagues at the Marine Biological Laboratory in

Woods Hole, Massachusetts, used genetic sequencing techniques which were not

available to give a more detailed view of complex organisms when Woese

began his work. They studied ribosomal RNA sequences of many eukaryotes,

including plants, animals, fungi and single-celled organisms. A statistical

analysis of genetic similarity showed that plants were the first to diverge

from a common stem, and that animals and fungi evolved later from a common

ancestor, similar to modern single-celled eukaryotes known as

choanoflagel-lates (Science, vol 260, p 340).

The relationship between the three types of multicelled organisms – fungi,

plants and animals – has long puzzled biologists. Fungi were first grouped

with plants, but were later classed in their own kingdom; their relationship

to plants and animals remained unclear. Woese put all three among the

eukaryotes, while making no attempt to resolve how they were interrelated.

He is ‘very enthusiastic’ about Sogin’s work.

Sogin says molecular genetics cannot pinpoint the moment when the groups

split, but other evidence indicates animals diverged from the other groups

about a billion years ago. His genetic data indicate the three share a

common lineage with choanoflagel-lates. The single-celled organisms have a

single flagel-lum surrounded by a ring of tentacles, and resemble collar

cells or choanocytes in sponges, which are among the simplest animals.