Restriction fragment length polymorphisms in ribosomal DNA were observ
ed within and between populations of Isotoma petraea and Macrozamia ri
edlei. Isotoma petraea is a diploid self-pollinating species which has
evolved complex hybridity in response to inbreeding and exhibits high
levels of genetic differentiation between its geographically isolated
populations. Macrozamia riedlei is a diploid dioecious wind-pollinate
d cycad with high levels of gene flow throughout its population system
. Variation in rDNA was observed within and between populations of bot
h species. In I. petraea the level of variation was low with most indi
viduals having one length variant, except in populations with fixed hy
bridity, whereas in nl, riedlei high levels of variation were observed
with an average of eight variants per individual. The pattern of vari
ation in I. petraea showed homogenization of length variants within in
dividuals and populations with divergence of length variants between p
opulations. The pattern of variation in M. riedlei showed mixing of le
ngth variants and maintenance of various length variants within indivi
duals and populations. The different patterns of variation are consist
ent with the different breeding systems of the two species.