Studies on mouth brooding cichlids of Lake Malawi: genetic,ecological and morphological differences

概要

Doctoral Thesis
Studies on mouth brooding cichlids
of Lake Malawi: genetic, ecological
and morphological differences
(Summary)

Shaharior Hashem
Department of Bioresource Science
Graduate School of Biosphere Science
Hiroshima University

March 2020

Summary of the Dissertation

In most recent couple of decades, East African cichlids have been studied as a classic
example of fast vertebrate evolution and biological diversification. A big number of
cichlid species have been identified from three great lakes of East Africa: Lakes Malawi,
Tanganyika, and Victoria. Malawian haplochromine cichlids are endemic, suggesting
that most of this diversification has taken place within the temporal and spatial
boundaries set by the lake shores. Examination of the genetic structure of recently
divergent populations or species is relevant to our understanding of the process of
speciation in general. Species richness and their divergence in morphology and trophic
adaptations make cichlids premier of rapid speciation. Hybridization between species or
genera is considered to have rapidly boosted genetic variation, which may enable
speciation and adaptive radiation.

Haplochromines are divided into 2 large groups. One is called ‘mbuna’, living on
rocky shores and feeding mainly on attached algae, and another is ‘non-mbuna’, living
in sandy shores or the ‘pelagic zone’ and feeding on a variety of items. Cichlid fishes
have evolved incredible morphological and behavioral diversity. Within Lake Malawi,
the phenomena of hybridization and retention of ancestral polymorphism explain allele
sharing across species. Egg-spots, particularly on male fins, are considered to have a
vital role in the courtship and spawning behavior of haplochromines. These spots
contain a transparent outer ring and a bright colored yellow, orange or reddish center. In
Asian countries, there may be some serious problems of genetic disturbance,
particularly in bred individuals. This disturbance was mainly caused by hybridization

particularly between species of genus Aulonocara or even between genera Aulonocara
and Sciaenochromis for improvement in commercial values in view of gorgeous
appearance and infection resistance.

The purpose of this study was to understand the genetic diversity and
relationships among the species of these genera in relation to large groups, mbuna and
non-mbuna, to examine genetic disturbance among wild and purely bred individuals and
those bred in Asia including Thailand, Taiwan and Japan with sequences of the
mitochondrial control regions, using haplotype network analysis, and to discuss the
cause of these disturbances. This study also investigates the relationships of genetic,
morphological and ecological characters of Malawian cichlids.

Chapter 1 provide the general background about the Malawian Cichlids and its
importance to understand the genetic, ecological and morphological differences. It also
stated the outline and purpose of this study.

Chapter 2 investigated the genetic differences within wild or bred individuals in a
large group of non-mbuna Malawian cichlid genus Aulonocara and a related genus
Sciaenochromis. A total of 40 individuals of 18 species or variations from genera
Aulonocara and Sciaenochromis with other 13 species from 11 genera, including both
mbuna and non-mbuna, as references were determined for the DNA sequence of the
mitochondrial control region. In a genetic tree, constructed with Kimura-2 parameter as
a distance by NJ method, many species of the genus Aulonocara constructed a clade
with mbuna while only 2 species constructed a clade with non-mbuna. Genus

Aulonocara created three groups among which one is created by only one species.
Individuals, bred in Japan, of A. hansbaenchi with no genetic diversity, constructed a
small congeneric clade, suggesting genetic homogeneity. In contrast, some individuals,
bred in Japan, of S. fryeri with a high genetic diversity, suggesting genetic heterogeneity.
Similarly, Sciaenochromis fryeri was a member of a clade by mbuna while S. sp.
nyassae was a member of a clade by non-mbuna. Among Aulonocara species, A. steveni
and A. sp. Lwanda were estimated to be relatively near to ancestors by network analysis.
Some individuals of A. hansbaenschi and S. fryeri, bred in Asia, were proven to be far
distant from purely bred ones. Individuals of an artificially made variation, Aulonocara
sp. tricolor, participated in different clades, showed a high genetic diversity. These
results suggest that the genera Aulonocara and Sciaenochromis might be generated by
hybridization between mbuna and non-mbuna, and that there are some genetic
disturbances within the species of these genera by human activities.

Chapter 3 investigated the genetic distance, diversity, and relationship of two
large groups of Malawian cichlids. For this purpose, a total of 78 species from 42
genera was determined for the DNA sequence of the mitochondrial control region. The
genetic tree was constructed with Kimura-2 parameter as a distance by NJ method, and
it showed that mbuna created only one group whereas non-mbuna created four groups
by single or multiple genera. Some genera, such as Campsochromis and Labeotropheus
with no genetic diversity within genus, constructed a small congeneric clade, suggesting
genetic homogeneity. In contrast, some genera, such as Otopharynx and Placidochromis
with high genetic diversity, participated in multiple groups including mbuna and nonmbuna, suggesting genetic heterogeneity. Non-mbuna species showed an apparently

high average genetic distance whereas mbuna species showed an apparently low
average genetic distance. It was suggested that non-mbuna have attained much higher
diversification than mbuna. This different diversification can be explained by a stronger
ecological selection exerted in sandy areas where there are a variety of possible foods in
contrast to a stronger sexual selection in rocky areas where attached algae are a main
food and female’s mate choice are very severe.

Chapter 4 investigated the relationship between genetic, ecological and
morphological characters of cichlid fishes, in relation to two large flocks, based on
different parameters by using the hierarchical clustering method. Euclidean distance
was calculated between each combination of species based on the standardized means
for the ecological and morphological parameters. Species diversity within groups is
calculated by the Shannon–Wiener diversity index (H') which measures not only the
number of species but how the abundance of the species is distributed in the community.
In cluster dendrogram, non-mbuna species clusters were connected at a relatively small
Euclidean distance. The diversity index (H') by the morphological characters is 0.94,
which showed higher homogeneity of genetic groups in the dendrogram. On the other
hand, the value by the ecological characters is 1.34, which showed higher heterogeneity
in terms of genetic groups. Thus, the morphological grouping showed a better match
with the genetic groupings than the ecological groupings. The clear difference between
these matchings suggests a similar pattern of effects of sexual or ecological selection on
morphology as well as genes.

Chapter 5 summarized the findings in this study. The result of the present
dissertation provides some very important information for the cichlid divergence. The
reason of widespread and extensive genetic disturbance of cichlids may be a continuous
and repeated production of low cost fry with the non-purely bred parents, sometimes
accompanied by a high rate of mutation. Genetic disturbances resulting from selfish
artificial crossbreeding and inbreeding should be controlled. In genetic relationships,
non-mbuna has attained much higher diversification by a stronger ecological selection
exerted in sandy areas where there are a variety of possible foods, whereas mbuna has
attained lower diversification with a stronger sexual selection in rocky areas where
attached algae are a main food. The morphological grouping showed a better match with
the genetic groupings than the ecological grouping on the basis of diversity analysis.
Future studies should investigate the relationships by examination of other regions of
DNA. Inclusion of additional species also will help to clarify the diversification level
between the two large groups and provide reliable genetic relationships and speciation
history. This study will make many contributions to clarification of the cichlid evolution
and divergence, and mechanisms for stable community structure.

Keywords: Cichlids, Sexual selection, Ecological Selection, Lake Malawi,
Mitochondrial DNA