speciation in fungi and its categories

1. Speciation in Fungi

3. Speciation
Process by which new species arise from existing species
Group of individual organisms that are capable of
interbreeding to produce fertile offspring in nature
OR
Population 1
(Species A )
Population 1
(Species A )Physical, genetic
driven forces
Isolates due to
some changes
No Reproduction X

4. How To Study Speciation in Plant Pathogens?
The completion of speciation is signaled by the existence of distinct
RIMs. Thus, understanding how species originate necessarily focuses on
understanding what biological features prevent gene flow.
Speciation in
Plant Pathogen
Classical studies
Genomic studies
Refer to the studies that have identified
RIMs between species
Involves genome-scale studies that
identify genomic features that are
involved in reproductive isolation or in
interspecies differences

5. Ascochyta fabae Ascochyta lentis
disease in fava
bean
disease in lentil
bean
X
Produced fewer ascospores per ascus than pure species crosses
indicating the existence of a postmating nonecological RIM.
Inoculation with hybrid ascospores (A. fabae × A. lentis) did not induce
disease in either host.
This suggests a fitness depression in the hybrids and the presence of extrinsic
postzygotic isolation, and demonstrates an important role of environmental
factors in keeping these species apart
Classical studies

6. Genomic studies
Studies of speciation have also used genome-wide data to identify
adaptations/traits involved in reproductive isolation and to uncover
demographic history.
The development of next-generation sequencing (NGS) has allowed for genome-
level comparisons and has led to the application of genetic mapping to a wide
variety of organisms, including plant pathogens
Mycosphaerella graminicola and M. graminicola S1 (pathogens of domestic
wheat and wild grass, respectively, sharing a most common recent ancestor)
by studying the level of protein coding divergence using levels of
synonymous and nonsynonymous polymorphisms
Stukenbrock et al addressed the question of how much gene flow occurs
between the sister species
Gene flow between M. graminicola and S1 seems to be extremely rare,
possibly because of the divergence of the mating type MAT1-2 protein
sequence in these two species

7. Speciation events are classified according to whether the two
new species start out geographically separated, partly
separated, or mixed together.
 Allopatric speciation–separate geographical areas
 Peripatric speciation (tiny isolated subpopulation)
 Parapatric speciation–adjacent areas with some gene flow
 Sympatric speciation–same area
Modes of speciation

8. Allopatric Speciation
Speciation pattern in which a physical barrier that
separates members of a population ends gene flow
between them
 Speciation that occurs because population leaves
in different places
 Genetic divergences then give rise to new species

9. Allopatric speciation
 Single interbreeding
population
Figure 5.2
 Population divided by a
barrier subpopulations
isolated

10.  The two populations
evolve independently,
diverge in their traits.
Figure 5.2
 Populations reunited
when barrier removed,
but are now different
enough that they don’t
interbreed.

11. Sympatric Speciation
Sympatric speciation is the process through which new species
evolve from a single ancestral species while inhabiting the same
geographic region.
 In sympatric speciation the probability of mating between two
individuals depend only on there genotype not on extrinsic
barriers (Kondrashov, 1986)

12. Sympatric Speciation in Wheat

13. Speciation
Species mode Comments
Ceratocystis fimbriata sensu Allopatric Fungi and hosts show disjoint ranges
lato
Botrytis cinerea/Botrytis Sympatric Authors equate ecological and sympatric speciation. They also
pseudocinerea clarify that allopatric speciation cannot be ruled out
Didymella rabiei, Sympatric The host species, Cicer spp., are sympatric in West Asia. Authors
phylogenetic species conclude that geographic proximity might reflect sympatric
divergence
D. rabiei, phylogenetic Undetermined Authors state they cannot discern between allopatric and
species sympatric divergence with the current evidence
Mycosphaerella graminicola Sympatric Authors argue that no geographical barriers separated pathogen
populations that occurred on wild grasses and populations that
adapted to the new domesticated crops. Coalescent analyses
based on six nuclear loci show little or no gene flow during the
divergence process
Phytophthora infestans and an Sympatric The current ranges of the two species overlap. Authors assume
undescribed novel no change in geographic range since speciation
population
Phytophthora Allopatric Authors suggest that secondary contact and hybridization (with
nicotianae/Phytophthora possible introgression) between these two species occurred after
cactorum an allopatric period of divergence
B. cinerea ecotypes Sympatric Authors argue that no geographical barriers separated pathogen
populations that occurred on wild grasses and populations that
adapted to the new domesticated crops. Cluster analysis of
microsatellites shows little or no gene flow during the
divergence process. Divergence is associated with host

14. Reproductive Isolation Mechanism
Speciation in sexually reproducing organism is the
decrease of gene flow between incipient species due to
the development of reproductive barriers.
 Pre-Zygotic reproductive barriers
 Post-Zygotic reproductive barriers
In fungi having a long dikaryotic stage, nuclear fusion
occur long after individual or gamete fusion, which
render the term post-zygotic ambiguous. We shall
therefore here use the term pre-mating and post mating
for fungi, which qualifies time before or after cell
division.

15. Fig. 12-15, p. 227
Different species
form and . . . Pre-zygotic mechanisms
Individuals reproduce at different times
(temporal isolation).
Physical incompatibilities prevent
individuals from interbreeding
(mechanical isolation).
Individuals live in different places so they
never meet up (ecological isolation).
Individuals ignore or do not get the
required cues for sex (behavioral
isolation).
Mating occurs
and . . .
No fertilization occurs (gamete
incompatibility).
Zygotes form
and . . .
Postzygotic mechanisms
Hybrid embryos die early, or new
individuals die before they can
reproduce (hybrid inviability).
Hybrid individuals or their offspring
do not make functional gametes
(hybrid sterility).
Interbreeding
is successful
Reproductive isolation
mechanism

17. What are the dominant drivers of divergence
and speciation in plant pathogens ?

18. Three possible scenarios in which reproductive isolation arises are shown.
Each color represents a different species.
 Geographical factors lead to the accumulation of genetic differences and
the buildup of reproductive isolation.
 Environmental factors, such as host or temperature, can lead to
reproductive isolation over time.
 In some rare instances, hybrid individuals might be reproductively isolated
from the parental species while still being fertile.
DRIVERS OF DIVERGENCE IN PLANT PATHOGENS
Geographical isolation Environmental Hybridization and speciation
In these cases, collectively referred as hybrid speciation, speciation does not
occur through divergence but instead occurs through reticulation.

19. Coalescent methods to recognize speciation processes
summarize the speciation history of two sister species, Sp1 and Sp2
m1 and m2
correspond to the
levels of migration
between the nascent
species
θA is the population size of
the ancestral population
θ1 and θ2 are the
population sizes
of the resulting
nascent species
θ1 > θ2
TMRCA is the time
to the most recent
common ancestor
according to the
gene genealogies
τ is the time that has
occurred since
speciation
TMRCA and τ are different in this example to account for intraspecific variation
in the ancestral population

20.  Evolutionary biology provides a powerful framework for
understanding key aspects of the natural history of pathogens.
 The study of evolutionary processes in plant pathogens is a
nascent field that promises to deliver new model systems and a
comprehensive view of mechanisms involved in speciation and
adaptation.
 On a more practical scale, the development of multilocus databases
will allow for the identification of pathogenic lineages in a
phylogeographic framework for additional comparative studies to
understand the population dynamics of plant pathogens.
 One of the ultimate goals of plant pathology is to be able to predict
the emergence of new pathogens, and this can be done using an
evolutionary perspective.
CONCLUSIONS

21.  The combination of classical approaches and high-resolution
genomic studies will reveal demographic and genetic characteristics
of how speciation occurs in oomycetes and fungal plant pathogens.
 Studying the evolutionary and eco-logical dynamics of plant
pathogens can lead to a better understanding of how these
organisms affect agricultural processes over time.

22. Reference
 Speciation in Fungal and Oomycete Plant Pathogens, Silvia Restrepo,
Javier F. Tabima, Maria F. Mideros, Niklaus J. Grunwald and Daniel R.
Matute, Annu. Rev. Phytopathol. 2014. 52:289–316
 Speciation in Fungi by Tatiana Guislaine Refregier, Mickael Le Gac,
Damien M. de Vienne, Michael E. Hood, Science Direct Fungal Genetics
and Biology 45 (2008) 791-802