After reading the paragraphs below, answer the questions that follow.
Plants differ from many animals because they undergo indeterminate (continual) growth. Indeterminate growth is common in clonal plants because a single clone can continually produce new clones without undergoing sexual reproduction, even if the plant is capable of doing so.
Senescence, an inability to undergo cell division as an organism ages, also occurs in many plants and animals. However, scientists have had a difficult time establishing the occurrence of senescence in cloning plants because of their indeterminate growth patterns. Mutations in somatic cells are common during asexual growth, and these mutations lead to a decrease in sexual fitness. Scientists hypothesize that over time, as these mutations accumulate, they may contribute to senescence in clonal plants. To test this theory, scientists studied a naturally growing population of trembling aspen (
Populus tremuloides), a tree species that grows clones from its root system but is capable of sexual reproduction. To determine if trembling aspens senesce, they compared genetic data of estimated tree age with the variable of "diminishing fertility": the amount of pollen released by male aspen trees. Here are the results for this part of the study:
Source: Adapted from Ally, D., Ritland, K., & Otto, S. P. (2010). Aging in a long-lived clonal tree.
PLoS Biol, 8(8): e1000454. doi:10.1371/journal.pbio.1000454.
With each successive generation of clones, the number of genetic mutations in the aspens' DNA increases, causing disruptions in pollen production. If mutation frequency were graphed per age of clones, what would the graph look like? Assume for each of these graphs that age increases moving from left to right on the
x-axis and the total number of accumulated mutations increases from bottom to top on the
y-axis.
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