Switchgrass - Bioenergy Crop Plants - page 566

Normal bivalent pairing has been reported for tetraploid and octaploid switchgrass plants (Riley
and Vogel 1982; Martinez-Reyna et al. 2001). Aneuploid variants and multivalent chromosome
associations are more frequent at higher ploidy levels (Barnett and Carver 1967; Brunken and Estes
1975). An analysis of segregation and linkage relationships for random markers distributed across
the genome of an upland × lowland tetraploid cross suggested polysomic inheritance (Missaoui
et al. 2005a). This conclusion was based on the observed ratios of single- to multiple-dose markers
and the observed ratios of loci linked in coupling vs. repulsion phase. Additional analyses, following
more complete genomewide marker saturation, clearly identified 18 linkage groups, suggesting an
all-tetraploid genome with disomic inheritance (Okada et al. 2010).
The evidence for preferential pairing and disomic inheritance within a polyploid series that
ranges from 2 n = 2 x = 18 to 2 n = 12 x = 108 is surprising. Normally, extensive polyploid series such
as this one arise by spontaneous doubling of whole genomes, leading to autopolyploidy (Mable
2003). The evidence for higher frequencies of aneuploid variants and multivalent pairing at higher
ploidy levels suggests multiple mechanisms of polyploidization, pairing, and gene inheritance
within switchgrass, i.e., apparent allopolyploidy and disomic inheritance at the tetraploid level
and possible autopolyploidy and polysomic inheritance at higher ploidy levels. It also raises the
intriguing question: If tetraploid switchgrass is an allopolyploid, what are its ancestors and, as
a corollary, do upland and lowland switchgrasses have different ancestors? Alternatively, is the
polyploid switchgrass genome in the process of diverging into two distinct and duplicate genomes?
Switchgrass is a member of the Paniceae tribe of grasses, which diverged from the Maydeae
(maize) tribe approximately 23 Mya. Upland and lowland ecotypes are thought to have diverged
from each other sometime between 1 and 2 Mya on the basis of sequences of the nuclear gene
encoding plastid acetyl-CoA carboxylase, Acc-1 (Huang et al. 2003). These authors further speculate
that polyploidization events were involved in this divergence, as the Acc-1 polymorphisms appear to
discriminate a “lowland tetraploid” from an “upland octaploid” form. Polymorphisms at the Acc-1
locus within tetraploid plants of both the upland and lowland ecotypes suggest the possibility of four
ancestral diploids (Huang et al. 2003).
Switchgrass has two cytoplasm types, “L” and “U” that are based on chloroplast DNA (cpDNA)
polymorphisms that are associated with the lowland and upland ecotypes, respectively, (Hultquist et al.
1996; Missaoui et al. 2006). The “L” cytoplasm types are tetraploids whereas the “U” types can be
either tetraploids or octaploids (Hultquist et al. 1996). Martinez-Reyna et al. (2001) used controlled
reciprocal crosses between “Kanlow” (“L” tetraploid) and “Summer” (“U” tetraploid) plants and a
restriction fragment length polymorphism (RFLP) marker to demonstrate that the chloroplast DNA of
switchgrass is maternally inherited. They also determined that the lowland and upland ecotypes and
associated cytoplasm types of switchgrass are completely cross-fertile at the tetraploid level and that
there is a high degree of similarity among their nuclear genomes as indicated by normal bivalent pairing
during meiosis. This is supported by linkage analyses and homology between “Alamo” (“L” tetraploid)
and Summer linkage groups (Missaoui et al. 2005a). Despite their homology, upland and lowland
ecotypes are genetically distinct, as demonstrated by cpDNA markers (Hultquist et al. 1996; Missaoui
et al. 2006) and random amplified polymorphic DNA (RAPD) markers (Gunter et al. 1996). Extreme
differential heading and anthesis dates, combined with physical isolation due to habitat differentiation
and fragmentation, are the likely causes of relatively recent genetic isolation of the upland and lowland
ecotypes. Upland and lowland ecotypes can occur sympatrically, with differential heading and anthesis
dates serving to maintain distinct and isolated germplasm pools (Brunken and Estes 1975).
Ecotypic variation in switchgrass derives largely from the broad geographic distribution of the
species, which extends from approximately 15 to 55° north latitude in North America. Photoperiod
is one of the principal drivers of ecotypic variation, ranging from 13 to 17 h on the summer solstice at
these extreme latitudes. Switchgrass is thought to have survived the Pliestocene Glaciation in three
refugia located in the southern United States, perhaps including northern Mexico: a western semi-
montane (dryland) region, a central humid region with rich, fertile soils, and a southeastern region,
perhaps near the Gulf Coast (McMillan 1959). With the retreat of glaciation approximately 11,000