Jd. Retief et al., EVOLUTION OF THE MONOTREMES - THE SEQUENCES OF THE PROTAMINE P1 GENESOF PLATYPUS AND ECHIDNA, European journal of biochemistry, 218(2), 1993, pp. 457-461
The protamine P1 genes from two monotremes, platypus (Ornithorhynchus
anatinus) and echidna (Tachyglossus aculeatus) were isolated after pol
ymerase-chain-reaction amplification then cloned and sequenced. The tw
o protamine P1 genes are of 290 bp and 311 bp for platypus and echidna
, respectively, and are clearly orthologous to the published sequences
of protamine P1 genes of eutherian mammals and birds. Both genes cont
ain an intron, like the mammals and marsupials and unlike the bird P1
genes that are intronless. The deduced protein sequences from the codi
ng areas of the platypus and echidna protamine P1 genes do not contain
any cysteine residues. This absence of cysteine residues leaves the s
perm nuclei susceptible to disruption in vitro by exposure to increasi
ng ionic strength and is a characteristic of fish, birds and marsupial
s. In contrast, the P1 protamines of placental mammals invariably cont
ain 6-9 cysteine residues that, as a result of the formation of interm
olecular and intramolecular disulfide bridges, significantly increase
the stability of the sperm nuclei that can only be disrupted following
disulfide-bond cleavage. Phylogenetic analysis of the protamine P1 ge
ne sequences indicates that the monotremes occupy a position half-way
between the eutherian mammals and birds. From the DNA sequences we est
imate the time of divergence of the platypus and the echidna to be aro
und 22 million years ago. This date agrees very well with the publishe
d estimates of divergence based on other criteria.