When a population becomes geographically isolated from the rest of its species, it may genetically diverge and ultimately become a new, reproductively isolated species.  In an early stage of this speciation process, hybrids between populations often show highly variable fitness, with some individuals showing fertility, survivorship and stress tolerances similar to the original parents, while others show higher or (more commonly) lower fitness.  Here our goal was to obtain a better understanding of hybrid fitness by examining both physiological traits of hybrids and their genetic composition (since each hybrid possesses a unique mix of the parental nuclear genomes and its mother's mitochondrial genome).  Studies focused on the copepod Tigriopus californicus which lives in pools on isolated rocky outcrops along the Pacific coast of North America.  Previous work on this species has found that fitness is frequently related to the performance of mitochondria, the intracellular organelles that synthesize most of the cell's energy in the form of ATP.  In this project, different T. californicus populations were hybridized in the lab and the resulting offspring were scored for various measures of fitness and then genotyped to assess the types of genomic interactions responsible for variable hybrid fitness.  The primary findings of these experiments were: 1) that hybrids with high fitness (measured by rate of development or survivorship) showed higher rates of ATP synthesis in isolated mitochondria, and 2) the genotypes of the high fitness hybrids showed a bias toward elevated frequencies of alleles at nuclear genes that were derived from the same population as the mitochondrial genome; when the mitochondria came from population 1, nuclear alleles from population 1 were favored over substantial portions of the genome (several chromosomes). These results strongly support the hypothesis that mitochondrial function is a consequence of the match between nuclear and mitochondrial genomes as is predicted by the intimate interactions between specific genes that form the ATP synthesizing machinery in the mitochondria.  Because essentially all eukaryotic cells share this mosaic nature of energy production (i.e., encoded in both nuclear and mitochondrial genomes), the demonstration that optimal function requires a matched set of genes has important implications across biology, ranging from conservation and evolutionary biology to human mitochondrial medicine.

In addition to its broader impacts for biology, the project has also contributed in the area of public scientific literacy.  Specific outreach projects have included 3 years of partnership with the Ocean Discovery Institute in providing a hands-on DNA barcoding exercise for 5th grade students (approximately 800 students annually) in underserved San Diego public schools, creation of a new exhibit at the Birch Aquarium at Scripps (which has over 400,000 visitors annually) describing how work on copepods informs general principles of response to ocean warming, and a public lecture on the biology of ocean warming to an audience of ~200 people at the Birch Aquarium; the latter is now available on YouTube where it has received over 19,000 views.

Last Modified: 07/06/2020
Modified by: Ronald S Burton