tained for the Ustilaginomycotina, but in microbo-
tryaceous smuts, hybrid inviability was shown to select
against hybrids (De Vienne et al. 2009 ).
InU. maydis, sorus formation is initiated by a combi-
nation of parasite and host effectors. To develop telios-
pores,U. maydisspecifically alters plant expression,
initiating different expression profiles in different host
tissues (Skibbe et al. 2010 ). These experiments demon-
strated that the interaction between smuts and their
hosts is extremely tight at the molecular level, which
suggests that there are strong factors in maintaining
boundaries between parasite species. Thus, species
concepts incorporating host information, as applied
by smut fungal taxonomists for the last century, have
a biological basis, which could explain such narrow
host ranges in smut fungi (Cai et al. 2011 ).
II. Life Cycle
Species of Ustilaginomycotina share a similar
dimorphic life cycle comprised of a saprobic
haploid phase and a parasitic dikaryotic phase
(Fig.11.2) (Brefeld 1883 ; de Bary 1884 ; Samp-
son 1939 ). The haploid phase is initiated usu-
ally by the formation of basidiospores following
meiosis of the diploid nucleus in the basidium
and ends with the conjugation of compatible
haploid cells to produce dikaryotic, infectious
hyphae. The dikaryotic phase ultimately results
in the production of probasidia (i.e. often tel-
iospores) or basidia (Fig.11.3a–o).
Almost all Ustilaginomycotina sporulate on
or in parenchymatic tissues of their hosts. In the
majority of the Ustilaginomycotina,the young
basidium becomes thick-walled and at maturity
separates from the sorus to function as a dis-
persal agent, the teliospore. Teliospores are usu-
ally the most conspicuous stage in a smut’s life
cycle, representing the smut syndrome (cf.
Fig.11.1a–p). Most of the Ustilaginomycotina
are dimorphic and produce a yeast or yeast-like
stage in the haploid phase and form hyphae
during the parasitic phase. However, there are
several variations from this generalized life cycle,
e.g. the occurrence of homothallism inAnthra-
coidea(Kukkonen and Raudaskoski 1964 )and
Exobasidium(Sundstro ̈m 1964 ), thelack of tel-
iosporesin the Microstromatales, Exobasidiales
and Ceraceosorales (Begerow et al. 2001 , 2002 ,
2006 ), or even the switch to a complete anamor-
phic life style as assumed forMalassezia(Boekh-
out et al. 2010 ) and other anamorphic genera.A. Saprobic PhaseMembers of Ustilaginomycotina can survive
outside their hosts during a free-living asexual
state, the saprobic phase.Many representatives
are readily obtained from nature as predomi-
nantly unicellular budding states, called
yeasts or sporidia, e.g. species in Ustilago,
Microstroma, andMalassezia(Begerow et al.Fig. 11.2Generalized life cycle of Ustilaginomycotina
Ustilaginomycotina 299