These cells have to divide only once to become fertilizable," Makova explained. "Females of a species are born with their entire lifetime supply of oocytes, or egg cells. Makova's team also found that generation time affects male mutation bias - a higher rate of DNA mutation in the male sperm versus the female egg. After comparing 32 mammalian species, her team found that the strongest, most significant life-history indicator of mutation rate was, in fact, the average time between a species member's birth and the birth of its first offspring, accounting for a healthy 40% of mutation-rate variation among species. "If we do the math we see that, for mice, every 100 years equates to about 200 generations, whereas for humans, we end up with only five generations every 100 years," Makova said. Humans, on the other hand, have offspring when they are at least in their mid-teens or even in their twenties, and thus have a longer generation time. On the one hand, mice in the wild usually have their first litter at just six months of age, and thus their generation time is very short. "The more generations a species has per unit of time, the more chances there are for something to go wrong that is, for mutations or changes in the DNA sequence to occur." Makova explained that the difference between mice and humans could be used to illustrate how vastly generation time can vary from species to species. "The expected relationship between generation time and mutation rate is quite simple and intuitive," Makova said. To find correlations between life history and mutation rates, the scientists first focused on generation time. Image credit: Anton Nekrutenko, Makova lab, Penn State University The elephant is one of the 32 mammal species, including human, whose life-history traits and DNA mutation rates are studied in the Penn State University laboratory of Kateryna Makova. "So, if we have information about how extant species' life history affects mutation rates, it becomes possible to make inferences about the life history of a species that has been extinct for even tens of thousands of years, simply by looking at the genomic data." "Correlations between life-history traits and mutation rates for existing species make it possible to develop a hypothesis in reverse for an ancient species for which we have genomic data, but no living individuals to observe as test subjects," Makova explained. One of the many implications of this research is that life-history traits of extinct species now could be discoverable. Credit: Anton Nekrutenko, Makova lab, Penn State University The wild dog is one of the 32 mammal species, including human, whose life-history traits and DNA mutation rates are studied in the Penn State University laboratory of Kateryna Makova. The results of the research will be published in early online edition of the journal Evolution on 13 June 2011. They then correlated their estimations with several indicators of life history. For each species, they studied the mutation rate, estimated by the rate of substitutions in neutrally evolving DNA segments - chunks of genetic material that are not subject to natural selection. The team of researchers led by Kateryna Makova, a Penn State University associate professor of biology, and first author Melissa Wilson Sayres, a graduate student, used whole-genome sequence data to test life-history hypotheses for 32 mammalian species, including humans. These traits include metabolic rate and the interval of time between an individual's birth and the birth of its offspring, known as generation time. For the first time, scientists have used large-scale DNA sequencing data to investigate a long-standing evolutionary assumption: DNA mutation rates are influenced by a set of species-specific life-history traits.
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