Tissue-specific compensatory mechanisms and erythropoiesis in triploid zebrafish

Abstract

In eukaryotes, cell size is proportional to the size of the nucleus and to the amount of genomic DNA housed within. Ploidy transitions, or Whole Genome Duplications (WGDs), double the amount of genomic DNA in the nucleus and cell size is increased consequently. Genome duplication is a driver for the evolution of novel traits by producing genetic redundancy that buffers the negative fitness consequences of mutations in highly conserved genes. WGDs have been discovered in nearly every major taxon of multicellular eukaryotes, but are more common in plants than in animals for reasons that are not clear. In plants, changes in ploidy and genome size changes the size of the organism as the number of cells allocated to tissues does not compensate for larger cells. In contrast, cell size and body size are decoupled in vertebrates so polyploids are no larger than concomitant diploids. Research on the evolution of polyploids usually focuses on the fate of duplicated genes rather than on the developmental and physiological consequences of changing cell size and the cellular granularity of tissues (the degree to which a tissue is subdivided into discrete cells). Perhaps the relative scarcity of WGDs in animals pertains more to how these developmental and physiological consequences affect fitness rather than the evolutionary consequences that occur in later generations after polyploids have successfully reproduced. Ploidy can be experimentally manipulated in most teleosts, but diploid/triploid comparisons are most common as triploids are reproductively sterile making them useful for the aquaculture industry. Triploids consistently underperform compared to diploids with reduced ability to tolerate stressful conditions and higher mortality rates when raised in conditions optimized for diploids. Triploids typically struggle to tolerate aerobic challenges leading many to suggest a cardiorespiratory limitation, but there is no clear mechanism explaining this difference between ploidies. The zebrafish is a popular model vertebrate that has been a boon for developmental biologists and, increasingly, physiologists. In this thesis, I have taken advantage of the many mutants and transgenic strains with tissue-specific fluorescent reporters to answer questions about the biology of polyploid vertebrates that would not be possible in classic, non-model species such as in Atlantic salmon. To do so, I developed a novel ploidy determination technique to non-lethally assess the amount of DNA in the nuclei of zebrafish embryos in vivo using confocal microscopy and image processing (Chapter 2). I demonstrated for the first time that the compensatory mechanisms maintaining organ and body size in polyploids are tissue-specific by comparing the morphology of the blood, muscle, and the vasculature at single cell resolution (Chapter 3). Focusing in on the blood and erythropoietic system, I quantified hematopoietic stem cells in diploid and triploid embryos to show that triploids have fewer of these blood progenitor cells in their hematopoietic tissue. Triploids also produce erythrocytes at a slower rate compared to diploids suggesting that there are more aged, senescent erythrocytes in circulation perhaps contributing to the well-documented physiological limitations of triploids compared to diploids (Chapter 4). The utility of the zebrafish system makes it an ideal organism for studying the biology of polyploids not only to improve the performance of triploids in aquaculture, but also to better understand why polyploidy is rarer in animals than in plants.