Exercise 9.2

Patterns of Speciation in Fungi

(This exercise is based on Le Gac, M., M. E. Hood, and T. Giraud. 2007. Evolution of reproductive isolation within a parasitic fungal species complex. Evolution 61: 1781–1787.)

(Note: The reference above links directly to the article on the journal’s website. In order to access the full text of the article, you may need to be on your institution’s network [or logged in remotely], so that you can use your institution’s access privileges.)


In 1989, Jerry Coyne and Allen Orr pioneered an approach to studying patterns of the evolution of reproductive isolation. From published studies, they took data on measures of genetic distance and indices of reproductive isolation from various pairs of Drosophila species and plotted these data in graphs. From analyses of these graphs, Coyne and Orr could determine whether aspects of reproductive isolation between species pairs correlated with the genetic distance between the species. They found that species that were more genetically distant tended to have both higher levels of prezygotic and postzygotic reproductive isolation. Assuming that genetic distance increased more or less steadily with time (i.e., there was a molecular clock, see Chapter 2), Coyne and Orr’s data provide a time course of the evolution of reproductive isolation in flies.

In the twenty years since Coyne and Orr’s seminal paper, multiple investigators have conducted similar analyses on a wide range of organisms, including frogs, birds, fish, butterflies and moths, and various genera of plants. Interestingly, some similar patterns are seen across many of these rather diverse taxa. For instance, reproductive isolation usually increases with genetic distance (and that rate is fairly similar in frogs and flies), and hybrid sterility often evolves at a faster rate than does hybrid inviability.

Until recently, no such studies had been performed in fungi. In 2007, Mickael Le Gac, Michael Hood, and Tatiana Giraud published their results from studies examining patterns of reproductive isolation in the fungal species complex Microbotryum violaceum. These fungi, which cause anther smut disease in a variety of plants, represent cryptic species that are morphologically similar but differ in their host associations. These researchers first used DNA sequences taken from three different genes to determine phylogenetic relationships of and genetic distances between the isolates.


Figure 1  The phylogenetic relationships of the M. violaceum isolates based on DNA sequence data taken from three different genes.


Question 1. Refer to Figure 1 above. Based on this phylogeny, is MvSl more closely, more distantly, or equally related to MvSn as it is to MvDsp1?


Question 2. Based on this phylogeny, is MvLf more closely, more distantly, or equally related to MvSd as it is to MvS1?

Understanding the types of reproductive compatibilities examined by the researchers requires knowledge of the life cycle of these fungi.

The anthers of infected plants produce diploid teliospores instead of pollen. After pollinators transport them to a new host, these teliospores go through meiosis. Haploid sporidia are the products of meiosis, and these have two mating types (a1 and a2), and hence are like gametes. Conjugation, which is equivalent to fertilization, occurs between sporidia of opposite mating types. After conjugation, the diploid zygote forms hyphae on the new host plant. If successful, the hyphal formation leads to systemic infection. Eventually, new diploid teliospores are produced, and the cycle begins anew.

Figure 2  The basics of the Microbotryum violaceum life cycle.


Question 3. Refer to Figure 2 above. Based on this life cycle, would reproductive isolation that limited the conjugation of heterospecific sporidia be considered prezygotic or postzygotic?

One component of reproductive isolation is how well pairings of sporidia from different isolates (heterospecific) conjugate. The researchers call this component, RI-1. Another component of reproductive isolation, RI-2, is how well the conjugated zygotes from heterospecific crosses form hyphae. Finally, RI-T measures how well the sporidia from heterospecific crosses infect the plant and produce teliospores. RI-T includes both RI-1 and RI-2, and is the most comprehensive measure of reproductive isolation.

The index for all three measures of reproductive isolation is calculated as follows. RI = 1 – (Ph/Pc) where Pc is the proportion of success in conspecific crosses and Ph is the percentage of success in heterospecific crosses. Note that if Pc = Ph, then RI = 0. So, if conjugation occurs successfully 40% of the time in a conspecific cross and 10% of the time in a heterospecific cross, RI-1 = 1 – (0.1/0.4), or 0.75.


Question 4. If hyphal formation is successful in 20% in the conspecific cross and 15% successful in the heterospecific cross, what is RI-2?


Question 5. Can a measure of RI be negative (less than zero). If so, under what conditions is it negative?

Figure 3  The extent of reproductive isolation for heterospecific crosses on the y-axis and genetic distance on the x-axis for (A) RI-1 (conjugation), (B) RI-2 (hyphal formation), (C) RI-T when S. latifolia is the host, and (D) RI-T when S. dioica is the host. Note that the scale is different in A than in B–D. Correlation coefficients and p values are also provided.


Question 6. Refer to Figure 3 above. Is there a significant relationship between RI-1 and genetic distance? If so, is it positive or negative?


Question 7. Is there a significant relationship between RI-2 and genetic distance? If so, is it positive or negative?


Question 8. Based on your responses to Questions 3, 6, and 7, what inferences can you draw about the relative rates of evolution of prezygotic and postzygotic reproductive isolation in these fungi?


Question 9. Are the patterns of evolution of RI-T strongly dependent on whether the host is S. latifolia or S. dioica?


Question 10. Considering the case when the host is S. dioica, at approximately what genetic distance would we expect RI-T to be 0.5?


Question 11. Assuming a molecular clock wherein genetic distance increases 0.05 every million years, after approximately how many years would we expect RI-T to equal 0.5?