Thursday, February 12, 2015

The Case of the Marvelous Meiosis

1. Meiosis in male (A) and female (B).
The typical story of meiosis is described in detail in many a biology textbook. It all starts with a diploid somatic cell. The chromosomes duplicate and homologs align along the cell's mid-line. The aligned chromosome doublets crossover and then the doublets are divided into two new cells (Meiosis I). The chromosome doublets align along the mid-lines and the new cells divide again (Meiosis II) to produce the final gametes. This model well matches the process of spermatogenesis (Fig 1A), which was used as a model to study and understand the process. Oogenesis follows basically the same process, except that each division is asymmetric. The result is one large gamete (Fig 1B) and two or three non-viable cell fragments with the extra chromosomes.

2. Meiosis in male (A) and female (B) Caninae group roses.
Roses in the group Caninae (Dog Roses) do a peculiar version of the process. The basic example is tetraploid, but produces haploid male (Fig 2A) and triploid female (Fig 2B) gametes. During meiosis I, two homologs for each chromosome align to form a bivalent and the rest remain as monovalents. The monovalents are lost during spermatogenesis, but retained during oogenesis. This system is referred to as "permanent odd polyploidy".  It even works with extra uneven chromosome copies. The extras form monovalents and are discarded or retained just like the red and blue chromosomes in Figure 2. This system allows fertility to be maintained even with odd chromosome counts that would normally make a plant sterile.

I came across this peculiar variant of meiosis while researching what complications might develop in a cross between Rosa pomifera (4n, group Caninae) and R. rugosa (2n). Both parent species have relatively large fruit. My initial plan was to hybridize the two species, generating at least one F1 plant, then allow them to self and screen many F2 progeny plants for increased fruit size.

That the two roses are not very closely related and both species have numerous smaller-fruited relatives suggests their large fruit was evolved separately. This is useful for a breeding project as it means that different mutations accumulated as each plant developed large fruit. Those separate mutations can possibly be recombined via hybridization to generate a progeny plant with even larger fruit.

I expect the project of domesticating roses as [more of] a fruit will take decades. It should be an amusing hobby and I expect to have enough time left to see some nice results.

  1. Meiosis:
  2. Rose groups:
  3. Dog rose meiosis: