Dietary adaptations in the intestinal tract of cichlids: macro and micro-morphological perspectives and phenotypic plasticity

Dietary adaptations play a pivotal role in evolutionary processes and contribute to the diversification of species. They involve changes at different levels of biological organisation that help organisms to exploit novel niches in a habitat, thereby reducing competition and improving food utilisation. Dietary adaptations have also been crucial in the rapid adaptive radiations of East African cichlids. While changes in the food up-take apparatus have been well documented in these closely related species, much less is known about the accompanying morphological, cellular and molecular variations in their intestinal tracts.
This doctoral thesis aims to understand the macro- and micro-morphological aspects of dietary adaptations in the intestines of Lake Tanganyika cichlids and investigate the role of phenotypic plasticity in adapting to various dietary regimes. To do so, we took a holistic approach, combining broad phylogenetic sampling and traditional histology methods with cutting-edge technologies including geometric morphometrics and single-cell transcriptomics, to investigate the morphological, cellular and molecular aspects of dietary adaptations in cichlids.
In the first chapter of the thesis, we first explored the size and shape diversity of abdominal cavities in cichlids. We noticed diet-related patterns in abdominal cavity morphology and found a strong correlation with the intestinal lengths of the respective species. Next, we analysed variations at the micro-morphological level, both along the intestinal axis and between species, by quantifying intestinal ridge patterns and various histoanatomical traits of the intestinal epithelium. Lastly, we performed correlation analyses between macro- and micro-morphological traits and ecological indicators, to decipher potential interrelations.
In the second chapter, we aimed to study to what extent these macro- and micro-morphological traits respond to changes in nutrient composition in plastic manner. To this end, we conducted common garden experiments in which fishes of different species and dietary categories were raised on experimental feeds of varying nutrient compositions. Varied responses in growth, intestinal length plasticity and histoanatomical traits to changes in dietary nutrients were observed. Finally, single-cell transcriptomic analyses of intestinal tissues revealed accompanying changes in cell type proportions and plastic responses at the level of gene expression between individuals from different diet treatment groups, underscoring the importance of phenotypic plasticity in responding to novel dietary sources.
Collectively, through phylogenetic-comparative sampling, interdisciplinary methods and integrative data analyses, the findings of this study broaden our understanding of the intestine’s role in dietary adaptations and, ultimately, organismal diversification.