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Abstract

The systematics of the Desmidiaceae is based upon comparative
iconography of cell morphology (alpha taxonomy) with current nomenclature
dating from 1848 with the publication of "The British
Desmidiaceae" by J. Ralfs. The widespread distribution of many
desmids, the diversity of form and the subjective element involved
in type specimens being recorded as iconotypes has produced a
cumbersome and often ambiguous alpha taxonomy.
In recent years there has been a trend towards a more natural
and biologically meaningful taxonomy (omega taxonomy) in higher
plants and animals as well as some algae. This, plus the problems
associated with desmid alpha taxonomy, instigated this investigation
of veg~tative and sexual morphology, genetic compatibility,
. and nuclear cytology in certain desmids. Most of these studies
involved the use of unialgal cultures under controlled environmental
condit~ons. In particular two Micrasterias species,
M. thomasicma and M. mahabuZeshwa:t'ensis arid related taxa were
chosen for their cosmopolitan distributions. This permitted comparisons
of the genetic compatibility underlying morphological
· uniformity,- or continua, on a- global ha-s i-s as well as in the -inore -
localized situation.
A study of wild and cultured cell morphology of these
Micrasterias species groups was followed by an investigation of
the processes and morphological aspects of sexual reproduction.
Sexual compatibility at both the intra-locality (within-population)
level and the_ inter-locality (between-populations) level was tested
by a series of crosses, zygospore production being taken as the
criterion of sexual compatibility. Genetic compatibility to the
extent of viability and fertility of the products of zygospore
germination (GPi) was also examined. In a limited number of cases
potential' gene flow between strains, which is the ultimate criterion
of the "biological species", was investigated by backcrosses of
zygospore products to parental strains. In addition, studies of meiosis and chromosome complements in germination vesicles were
undertaken to determine whether a cytogenetic basis existed for
taxonomic delimitation.
Similar studies were made on a number of Pleurotaenium strains
spanning several alpha taxa. However, mitosis in vegetative cells
and not meiosis was investigated for selected strains.
Finally a cursory examination of some desmids from tropical
Northern Australia was made to investigate diversity and sexual
reproduction in the special ecological conditions of this region.
These investigations revealed a genetic complexity within
.alpha taxonomic species, far exceeding morphological plasticity.
Complex patterns of sexual and genetic compatibility within a
single locality contrasted with compatibility between strains from
the United Kingdom and Australia, and North America and Australia.
At the cytological level genetic compatibility was often associated
with widely differ~ng chromosome complements, even within a single
strain.
The results are discussed with respect to possible dispersal·
and evolutionary patterns in desmids. It is concluded that at
..
this stage· alpha taxonomy must be retained for the desmids but
that a more realistic approach to desmid systematics ought to be
possible when aided by a knowledge of the genetic variability and
compatibility.

Item Type:	Thesis - PhD
Authors/Creators:	Blackburn, SI
Additional Information:	Copyright the Author
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