Travel Reference
In-Depth Information
bacteria, swept from their carefree lives into the ruminant's digestive stom-
ach, meet a swift fate as the newly activated lysozyme that is carried along
with them chews through their cell walls and releases their eminently digest-
ible contents.
Primates also have the gene for this stomach type of lysozyme. Although
we humans and the other omnivorous primates make only small amounts of
this lysozyme, the gene has been turned on full-blast in fermenting primates.
Wilson found that the stomach genes of monkeys and cows resemble
genes from any mammal are distinctly dif erent from tears-and-saliva genes.
He concluded that an ancestor of the two types of gene had duplicated in the
distant past, and the duplicates had begun to diverge to take up their separate
functions even before the early divergence of the monkey and cow lineages.
The lysozyme in most birds behaves like the mammalian tears-and-saliva
enzyme. The bird lysozyme is not secreted in the crop and takes no part in
digestion. But it does make up about 3% of the whites of bird eggs. The egg
lysozyme is active at the slightly alkaline pH typical of eggs and other tissues.
This activity explains why eggs remain bacteria-free as the chicks develop.
Bird egg lysozyme is a highly ef ective antibacterial agent. All bird lysozyme
genes are extremely similar to each other and are quite distinct from the
genes of mammalian lysozymes.
Hoatzin egg white lysozyme behaves like other bird lysozymes. But hoatz-
ins have a total of fi ve lysozyme genes, which arose through a process of gene
duplication during the long period when the ancestors of hoatzins evolved
on a separate lineage from other birds. Like the claws on the wings of hoatzin
chicks, these duplicated genes have not been found in other living birds.
Three of these duplicated hoatzin genes have evolved away from the
typical bird gene, and now code for the lysozymes that hoatzins secrete into
their crops.
The forms of lysozyme secreted into the hoatzin crop behave dif er-
ently from the normal bird lysozyme, and more like the mammalian stom-
ach lysozyme. They are inactive under the typical alkaline conditions of the
hoatzin crop, but become active under the acidic conditions found in the
digestive stomach and in the gut.
