Question

You are interested in finding out how the budding yeast Saccharomyces cerevisiae is so good at making bread and have collected five new related species from the wild. You sequence the genomes of all of these new species and also consult with a fungal biologist to help you construct the phylogenetic tree shown in Figure Q9-29. You find that species V, W, and X make pretty good bread whereas species Y and Z do not, suggesting that the last common ancestor of species X and S. cerevisiae may have the genes necessary for making good bread. You compare the gene sequences of species X and S. cerevisiae and find many identical coding sequences, but you also identify nucleotides that differ between the two species. Which species would be the best to examine to determine what the sequence was in the last common ancestor of species X and S. cerevisiae?

(a) species V
(b) species W
(c) species Y
(d) species Z

Answer 1

Species W is the best candidate to examine when comparing its genome to those of species X and Saccharomyces cerevisiae, aiming to identify ancestral sequences that contribute to the ability to make good bread.

Step-by-step explanation:

To identify the sequences in the last common ancestor of species X and Saccharomyces cerevisiae (important for making good bread), we should examine the species that is most closely related to both X and S. cerevisiae but isn’t one of them.

Given that species V, W, and X are capable of making pretty good bread, with species W being more closely related to S. cerevisiae than species V is, species W or V could provide useful comparative genomic information.

However, if species W is more closely related to S. cerevisiae than species X, then species W would likely have diverged later and thus be the best candidate to study to determine the ancestral genetic sequence. Consequently, the correct answer would be (b) species W.