Selection for Mitochondrial Quality Drives Evolution of the Germline

<div><p>The origin of the germline–soma distinction is a fundamental unsolved question. Plants and basal metazoans do not have a germline but generate gametes from pluripotent stem cells in somatic tissues (somatic gametogenesis). In contrast, most bilaterians sequester a dedicated germline early in development. We develop an evolutionary model which shows that selection for mitochondrial quality drives germline evolution. In organisms with low mitochondrial replication error rates, segregation of mutations over multiple cell divisions generates variation, allowing selection to optimize gamete quality through somatic gametogenesis. Higher mutation rates promote early germline sequestration. We also consider how oogamy (a large female gamete packed with mitochondria) alters selection on the germline. Oogamy is beneficial as it reduces mitochondrial segregation in early development, improving adult fitness by restricting variation between tissues. But it also limits variation between early-sequestered oocytes, undermining gamete quality. Oocyte variation is restored through proliferation of germline cells, producing more germ cells than strictly needed, explaining the random culling (atresia) of precursor cells in bilaterians. Unlike other models of germline evolution, selection for mitochondrial quality can explain the stability of somatic gametogenesis in plants and basal metazoans, the evolution of oogamy in all plants and animals with tissue differentiation, and the mutational forces driving early germline sequestration in active bilaterians. The origins of predation in motile bilaterians in the Cambrian explosion is likely to have increased rates of tissue turnover and mitochondrial replication errors, in turn driving germline evolution and the emergence of complex developmental processes.</p></div

Mitochondrial segregation undermines adult fitness.

<p>Adult fitness is a function of zygote fitness (the mutation load inherited), mutational input, and random segregation during development. <b>(a)</b> In organisms with no tissue differentiation, adult fitness is similar to zygote fitness, as the number of new mutations accumulating within a single generation is limited, and variance in mutant load between cells within a tissue has no effect on adult fitness. <b>(b)</b> In organisms with early differentiation of multiple tissues, adult fitness is undermined by segregational variance, as some tissue-precursor cells receive a higher mutant load than others, and adult fitness depends on the function of the worst tissue. <b>(c)</b> Increasing the number of mitochondria decreases the variance in mutant load between tissue-precursor cells, and so reduces the loss in adult fitness caused by random segregation. Parameter values <i>μ</i>_S = 0.01, <i>μ</i>_B = 0.005, ten cell divisions to adulthood, and a lifespan equivalent to 40 cell division cycles. Underlying data can be found at: <a href="https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData" target="_blank">https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData</a>.</p

Oogamy improves adult fitness by temporarily suppressing variance.

<p><b>(a)</b> The mitochondria in the zygote are partitioned into daughter cells at each cell division. With isogamy (<i>Q</i> = 0, blue), the zygote contains the same number <i>M</i> mitochondria as normal somatic cells. In contrast, with oogamy (<i>Q</i> = 4, red) the larger number of mitochondria contained in the zygotes (2^<i>Q</i><i>M</i> = 800) are partitioned without further replication (over four rounds of cell division) until the standard mitochondrial number (<i>M</i> = 50) is restored. <b>(b)</b> A large oocyte (<i>Q</i> = 4, red) suppresses the variance in mutation load (<i>m</i>/<i>M</i>) in the first few rounds of cell division (left inset) compared with a small oocyte (<i>Q</i> = 0, blue). This early difference in variance is virtually lost after 20 rounds of cell division (right inset). Segregation is modelled as described in the Methods without further accumulation of mutations. <i>M</i> = 50 and for illustrative purposes the mutation frequency set at <i>m</i> = 25. <b>(c)</b> The early reduction in variance produced by oogamy improves adult fitness in organisms with multiple tissues but has practically no effect when there is no tissue differentiation. Mutation rates are set to <i>μ</i>_S = 0.01 and <i>μ</i>_B = 0.005, and the number of mitochondria to <i>M</i> = 50. The initial mutant load in the zygote is set to 20% (i.e., 2^<i>Q</i><i>M</i>/5). <b>(d)</b> The fixation probability (95% confidence intervals) of an allele <i>A</i> specifying oogamy <i>Q</i> depends on the number of somatic tissues. Increasing <i>Q</i> reduces variance in mutant load between tissues, improving somatic fitness (c), but decreases variance among gametes, reducing the efficacy of purifying selection. Moderate levels of mitochondrial oogamy are therefore expected to evolve more readily in organisms with high levels of somatic differentiation. Mutation rates are set to <i>μ</i>_S = 0.01 and <i>μ</i>_B = 0.005, and the number of mitochondria to <i>M</i> = 50. The dashed line indicates the fixation probability of a neutral mutant. Underlying data can be found at: <a href="https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData" target="_blank">https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData</a>.</p

Germline evolution depends on mitochondrial mutation rate.

<p><b>(a)</b> Heat map showing fixation probability of an allele encoding early germline sequestration (at generation <i>N</i>_<i>G</i> = 3) in a simple organism that lacks tissue differentiation and produces gametes by somatic gametogenesis (at generation <i>N</i>_<i>G</i> = 10), in relation to the rate of copying errors (<i>μ</i>_S) and background damage (<i>μ</i>_B). The early germline mutation is introduced at a frequency of 0.05 (see <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2000410#sec008" target="_blank">Methods</a>). Early germline sequestration is favoured by higher <i>μ</i>_S and lower <i>μ</i>_B (blue, top left). The early germline allele is selected against in organisms with low <i>μ</i>_S and high <i>μ</i>_B (red, bottom right), conditions that instead favour somatic gametogenesis. The solid line represents neutrality. <b>(b)</b> Increasing the number of tissues to eight makes it harder to fix an early germline (<i>N</i>_<i>G</i> = 3)—the region shaded in red expands (solid line versus dotted line) so germline fixation now requires higher <i>μ</i>_S and lower <i>μ</i>_B compared with (a). <b>(c)</b> Increasing the number of mitochondria to 200 in an organism with eight tissues has little effect on early germline sequestration. Thus, increasing the level of complexity (more tissues and mitochondria) does not favour early germline sequestration. Underlying data can be found at: <a href="https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData" target="_blank">https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData</a>.</p

Mitochondrial oogamy opposes early germline sequestration and requires additional germline cell divisions.

<p><b>(a)</b> Fixation probability (95% confidence intervals) of the allele <i>g</i>, specifying sequestration of an early germline at <i>N</i>_<i>G</i> = 3 cell divisions, decreases with increasing mitochondrial oogamy, especially in organisms with multiple somatic tissues. <b>(b)</b> The early germline at <i>N</i>_<i>G</i> = 3 is more likely to fix in organisms with multiple tissues if there are additional germline cell divisions that restore segregational variance between gametes. Parameter values <i>μ</i>_S = 0.01, <i>μ</i>_B = 0.005, <i>M</i> = 50. The dashed lines indicate the fixation probability of a neutral mutant. Underlying data can be found at: <a href="https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData" target="_blank">https://github.com/ArunasRadzvilavicius/GermlineEvolution/tree/master/FigureData</a>.</p

Life cycle of the model multicellular organism.

<p><b>(a)</b> Development from zygote (left-hand side) to adult, showing early tissue differentiation (cells with differing shades) and late formation of gametes (G) from somatic cells in the adult (somatic gametogenesis). <b>(b)</b> Equivalent multicellular development depicting sequestration of gametes early in development (early germline). Dotted line indicates further development; adult and gamete cells not drawn to scale. <b>(c)</b> Copying errors (<i>μ</i>_S) during replication of mitochondrial DNA. <b>(d)</b> Mutations caused by background damage (<i>μ</i>_B) from, e.g., ultraviolet (UV) radiation or reactive oxygen species (ROS). <b>(e)</b> Doubling followed by random segregation of mitochondrial mutants (red) at cell division increases variance between daughter cells. <b>(f)</b> Concave fitness function, in which cell fitness declines non-linearly with the accumulation of mitochondrial mutations (<i>μ</i>_S + <i>μ</i>_B) as seen in mitochondrial diseases (see text).</p

Basal protrusions mediate spatiotemporal patterns of spinal neuron differentiation

During early spinal cord development, neurons of particular subtypes differentiate with a sparse periodic pattern while later neurons differentiate in the intervening space to eventually produce continuous columns of similar neurons. The mechanisms that regulate this spatiotemporal pattern are unknown. In vivo imaging in zebrafish reveals that differentiating spinal neurons transiently extend two long protrusions along the basal surface of the spinal cord before axon initiation. These protrusions express Delta protein, consistent with the hypothesis they influence Notch signaling at a distance of several cell diameters. Experimental reduction of Laminin expression leads to smaller protrusions and shorter distances between differentiating neurons. The experimental data and a theoretical model support the proposal that neuronal differentiation pattern is regulated by transient basal protrusions that deliver temporally controlled lateral inhibition mediated at a distance. This work uncovers a stereotyped protrusive activity of newborn neurons that organize long-distance spatiotemporal patterning of differentiation

BMP Signaling Gradient Scaling in the Zebrafish Pectoral Fin

Secreted growth factors can act as morphogens that form spatial concentration gradients in developing organs, thereby controlling growth and patterning. For some morphogens, adaptation of the gradients to tissue size allows morphological patterns to remain proportioned as the organs grow. In the zebrafish pectoral fin, we found that BMP signaling forms a two-dimensional gradient. The length of the gradient scales with tissue length and its amplitude increases with fin size according to a power-law. Gradient scaling and amplitude power-laws are signatures of growth control by time derivatives of morphogenetic signaling: cell division correlates with the fold change over time of the cellular signaling levels. We show that Smoc1 regulates BMP gradient scaling and growth in the fin. Smoc1 scales the gradient by means of a feedback loop: Smoc1 is a BMP agonist and BMP signaling represses Smoc1 expression. Our work uncovers a layer of morphogen regulation during vertebrate appendage development

Morphogen gradient scaling by recycling of intracellular Dpp

Morphogen gradients are fundamental to establish morphological patterns in developing tissues1. During development, gradients scale to remain proportional to the size of growing organs2,3. Scaling is a universal gear that adjusts patterns to size in living organisms3–8, but its mechanisms remain unclear. Here, focusing on the Decapentaplegic (Dpp) gradient in the Drosophila wing disc, we uncover a cell biological basis behind scaling. From small to large discs, scaling of the Dpp gradient is achieved by increasing the contribution of the internalized Dpp molecules to Dpp transport: to expand the gradient, endocytosed molecules are re-exocytosed to spread extracellularly. To regulate the contribution of endocytosed Dpp to the spreading extracellular pool during tissue growth, it is the Dpp binding rates that are progressively modulated by the extracellular factor Pentagone, which drives scaling. Thus, for some morphogens, evolution may act on endocytic trafficking to regulate the range of the gradient and its scaling, which could allow the adaptation of shape and pattern to different sizes of organs in different species