BACKGROUND: Gene conversion is the mechanism proposed to be responsible for the homogenization of multigene families
such as the nuclear ribosomal gene clusters. This concerted evolutionary process prevents individual genes in gene clusters
from accumulating mutations. The mechanism responsible for concerted evolution is not well understood but
recombination during meiosis has been hypothesized to play a significant role in this homogenization. In this study we
tested the hypothesis of unequal crossing over playing a significant role in gene conversion events within the ribosomal
RNA cistron during meiosis, mitosis or both life stages in the fungal tree pathogen Ceratocystis manginecans.
METHODS: Ceratocystis manginecans, a haploid ascomycete, reproduces homothallically and was found to have two distinct
sequences within the internally transcribed spacer (ITS) region of the ribosomal RNA cistron. The different ITS types were
scored using PCR-RFLP assays and chi-square analyses to determine the level of significance of the changes in the ratios of
the ITS types.
RESULTS: The relative ratios of the two ITS sequence types changed when the fungal isolates were cultured vegetatively or
allowed to produced sexual structures and spores. These active changes were shown to occur more frequently during
meiosis than mitosis.
CONCLUSION: The evidence presented provides concrete support for homogenization in the rRNA gene clusters found in this
fungus and that the most reasonable explanation for this process is unequal crossing over.
Figure S1 Aligned DNA sequence. Screen print showing the
aligned DNA sequences of only ITS sequence type Y in Ceratocystis
manginecans isolates CMW 13581, 13584, 23641 and 23643.
Figure S2 Aligned DNA sequence. Screen print of aligned
DNA sequences showing the differences in the sequences of the
two ITS sequence types (Z and Y) in Ceratocystis manginecans CMW
Figure S3 Hypothetical rRNA cistron showing meiotic
and mitotic divisions. Illustration of a hypothetical situation
for Ceratocystis manginecans where we assume that its rRNA cistron
has 600 copies undergoing meiotic and mitotic divisions. In this
example, under meiotic conditions each division represents either
an increase or a decrease in the ITS sequence types. Figure S3A
depicts a restricted cistron size scenario whilst Figure S3B has no
Table S1 Summary of the observed ITS sequence types
from 60 cloned amplicons across all test replicas for
both meiosis and mitosis based on five sequential
rounds of either mitotic transfers or sexual crosses for
Conceived and designed the experiments: KN BDW. Performed the
experiments: KN. Analyzed the data: KN BDW MPAC ETS MJW.
Contributed reagents/materials/analysis tools: BDW KN. Wrote the
paper: KN BDW MPAC ETS MJW.