Yes, Saccharomyces pastorianus (S. pastorianus or simply lager yeast) is the result of sexual reproduction. S. pastorianus is an interspecies hybrid. At the moment, one parent appears to be the Patagonian yeast species Saccharomyces eubayanus (S. eubayanus). The other parent is a yet to be identified strain of Saccharomyces cerevisiae (S. cerevisiae or simply ale, baker’s, or wine yeast) that was more than likely already used in brewing in Europe.
Most yeast cell reproduction is asexual via budding. Reproduction via budding is a biological process known as mitosis. During mitosis, a complete copy of the parent cell’s genome is passed to the child cell. In effect, the child cell is a clone of the parent cell. However, in sexual reproduction, a biological process known as meiosis occurs. Meiosis is a type of cell division that results in child cells that only possess half of their parent's genome. These cells are what are known as haploid cells, meaning that they only have one set of chromosomes. Put into human reproductive terms, sperm cells and eggs are haploid cells, each of which contains one set of twenty-three chromosomes. When a sperm cell and an egg combine, they produce a diploid cell that contains two sets of twenty-three chromosomes. In the case of yeast cells, haploid yeast cells contain sixteen chromosomes whereas diploid yeast cells contain two sets of sixteen chromosomes.
Now, what is really interesting about an interspecies hybrid brewing yeast strain like S. pastorianus is that it is almost a complete freak of nature. Most brewing strains are polyploids that either do not sporulate or sporulate poorly. Polyploids are cells that contain more than two sets of chromosomes, with most brewing strains falling into the triploid (three sets of chromosome) and tetraploid (four sets of chromosomes) classifications. To further complicate matters, a condition known as aneuploidy is very common in brewing strains. Aneuploidy is the result of the loss or addition of a chromosome that results in the total number of chromosomes not being a multiple of 16. Down Syndrome is an example of aneuploidy in humans. Children with Down Syndrome have an extra copy of chromosome 21 (i.e., children with Down Syndrome have 47 chromosomes instead of 46).
With that said, what I find completely fascinating about S. pastorianus is that the Saaz and the Frohberg families are polyploids. The latest research from Carlsberg Laboratory hypothesizes that the Saaz strains are triploids (a.k.a. allotriploids) with one set of S. cerevisiae chromosomes and two sets of S. eubayanus chromosomes whereas Frohberg strains are tetraploids (a.k.a. allotetraploids) with two sets of S. cerevisiae chromosomes and two sets of S. eubayanus chromosomes. This research also points to a single hybridization event instead of two separate events. The research group at Carlsberg believes that the loss of the S. cerevisiae genetic contribution in the Saaz family is the result of environmental stress. Their hypothesis is that Danish and Czech brewers historically fermented at lower temperatures than German and the Dutch brewers. The use of lower primary fermentation temperatures resulted in the loss of S. cerevisiae genes, as S. pastorianus inherited its cold tolerance genes from S. eubayanushttp://www.g3journal.org/content/early/2014/02/26/g3.113.010090.full.pdf