Chloroplast genome evolution in New Zealand mycoheterotrophic Orchidaceae : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Plant Biology at Massey University, Manawatu, New Zealand
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Date
2019
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Massey University
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Abstract
The plastid genomes, or plastomes, of most photosynthetic land plants are highly similar. In
contrast, those of non-photosynthetic, heterotrophic land plants are often reduced in both size
and gene content. The apparent degradation of mycoheterotrophic plant plastomes has been
attributed to a functionally-driven stepwise pattern of loss. However, the number of complete
plastome sequences available for mycoheterotrophic plants is small and taxonomic coverage
is biased. In this thesis, the plastomes of two mycoheterotrophic orchid species endemic to
New Zealand, Corybas cryptanthus Hatch (Diurideae) and Danhatchia australis Garay &
Christenson (Goodyerinae), as well as those of an albino and several photosynthetic
representatives of Corybas are reported. Beyond increasing the number of
mycoheterotrophic plastomes available for evaluating broad hypotheses about plastome
evolution in non-photosynthetic plants, these data also provide insights into two little studied
aspects of plastome evolution in mycoheterotrophs; intraspecific variation in the plastomes
of mycoheterotrophs and the differences between mycoheterotrophs and their closest
photosynthetic relatives.
The plastomes of C. cryptanthus and D. australis differ in the extent to which they
are degraded. Perhaps unexpectedly, the plastome of C. cryptanthus, which has close
photosynthetic relatives and therefore is likely to have arisen more recently than the
taxonomically isolated D. australis, is more reduced. Specifically, the plastomes of
C. cryptanthus are approximately half the size and have half the gene content of the other
Corybas sequenced whereas the plastome of D. australis is similar to those available for
photosynthetic relatives. This contrast may reflect underlying differences between the two
genera; the photosynthetic relatives of D. australis have plastomes containing NADH
dehydrogenase (ndh) genes whereas those of photosynthetic Corybas have lost their ndh
genes and their small single copy regions are highly reduced. These features may have
predisposed the ancestor of C. cryptanthus to rapid genome degradation. Finally,
observations on these results strongly suggest that plastome degradation follows, rather than
precedes, the shift to mycoheterotrophy.
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Keywords
Orchids, Genome mapping, Evolution, New Zealand, Chloroplasts