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

To discover the ancestral gene sequence for bread-making abilities in Saccharomyces cerevisiae and species X, one should examine the genes of species Y, as it is the closest relative that does not exhibit the trait.

Step-by-step explanation:

To determine the gene sequence in the last common ancestor of species X and Saccharomyces cerevisiae, the best approach is to examine the sequences of the species that are closest to the last common ancestor on the phylogenetic tree but not part of the group that exhibits the trait in question - in this case, the ability to make good bread. Since species V, W, and X can all make pretty good bread, they are closer to S. cerevisiae in terms of this trait. Species Y and Z do not make good bread, thus they are not as closely related in this characteristic.

However, species Y is more closely related to the last common ancestor of X and S. cerevisiae than species Z is. Therefore, examining species Y would likely give the best indication of the ancestral gene sequences.

Answer 2

The best species to determine the ancestral gene sequence for making good bread related to Saccharomyces cerevisiae, based on phylogenetic proximity and characteristics, would be either species Y or Z.

Step-by-step explanation:

You're looking to identify the best species for determining the ancestral gene sequence shared by species X and Saccharomyces cerevisiae for making good bread. To find ancestral gene sequences, you should compare the genomes of species that diverged just after the last common ancestor. Hence, the species most closely related to, but not included in, the group in question would be the best reference.

In this case, species Y and Z do not make good bread, suggesting that they branched off before the trait for good bread-making evolved. Among these, the species that is the next branch after the last common ancestor of species X and Saccharomyces cerevisiae would be the most informative for determining the ancestral sequence.

Without the figure, it is challenging to provide the mentioned correct option in final answer, but based on the information provided, the best candidate seems to be either species Y or Z, and not species V or W which are closer relatives to the effective bread-making strains.