Building on the notions of polyploidy and hybridization discussed above, it becomes possible to begin a proposed evolutionary history for this group of grasses. Paramount to such proposals is information arising from cytological evaluations (chromosome number and morphology, as well as extrachromosomal elements like satellite DNA) and hybridization studies of species within and between genera. If we assume, as most grass taxonomists will argue, that diploidy is the ancestral condition, then the relationship between Festuca scariosa (a diploid species endemic to the Sierra Nevada region of Spain) and F. mairei (a tetraploid), as enunciated by Borrill (1972), is key. Festuca scariosa is adapted to a dry, arid climate and has evolved quite distinctive morphological characteristics, such as reduced, highly lignified vascular strands. The 4x F. mairei shares this vascular characteristic and, in general, strongly resembles F. scariosa and can hybridize with it. Therefore, it can reasonably be deduced that F. mairei contains at least one genome derived from F. scariosa. The resemblance of the Moroccan polyploids (8x and 10x L. arundinaceum) to both 4x F. mairei and 2x F. scariosa further suggests the presence of a "scariosa" genome in these grasses. Despite the proposed evolutionary relationship between F. scariosa (section Scariosae) and the Moroccan polyploids (section Bovinae = subg. Schedonorus), it is evident that there is now considerable divergence between sections Scariosae and Bovinae. Recent molecular data support this distinction (Gaut et al., 2000) and, together with hybridization studies, further support the inclusion of F. mairei in section Bovinae.
The second key relationship is between 4x L. arundinaceum (F. arundinacea subsp. fenas) and the European 2x L. pratense. The latter species (Fig. 2-11) has a fairly wide geographic range and is abundant in meadows and low-lying pastures. Its range overlaps that of both 4x and 6x L. arundinaceum, which are very similar morphologically to L. pratense. Lolium pratense is also interfertile with 4x L. arundinaceum, and half of the chromosome complement of the latter pairs effectively with that of L. pratense. Consequently, L. pratense or a close relative is believed to have contributed a genome to 4x L. arundinaceum. Interestingly, both 4x L. arundinaceum and 4x F. mairei are considered allotetraploids and may in fact share a common genome. However, the contributor of the second genome to each of these species clearly is different and remains unknown in both cases.
As Borrill (1972) pointed out, the rest of the polyploids can be derived from the 2x and 4x species. Based on chromosome morphology (including the presence of satellite DNAs), 6x tall fescue (specifically, the northern ecotypes) likely arose from hybridization between 2x L. pratense and 4x L. arundinaceum. This would have given a sterile 3x hybrid, and a subsequent chromosome-doubling event generated the fertile 6x grass. Chandrasekharan and Thomas (1971) supported this view, finding close morphological similarities between 6x tall fescue and an F1 hybrid created by crossing L. pratense with 4x L. arundinaceum, then doubling the chromosome set. Normal meiotic pairing was observed in these 6x hybrids. Additionally, Humphreys et al. (1995) and Pasakinskiene et al. (1998) used DNA hybridization techniques and found that indeed, of the six chromosome sets in 6x L. arundinaceum, four appeared to be from 4x L. arundinaceum and two from L. pratense. However, as discussed below (see Evolutionary Origins of Hexaploid Tall Fescue section in this chapter), the heterogeneity among populations of 6x tall fescue may indicate multiple origins with different progenitor species.
Interspecific hybrids in the subgenus also predominate in North Africa. Chandrasekharan and Thomas (1971) and Chandrasekharan et al. (1972) bred F. mairei with 4x L. arundinaceum and doubled the chromosomes to get fertile 8x hybrids that were very good imitations of the naturally occurring 8x L. arundinaceum (F. arundinacea subsp. atlantigena). Their cytological results in fact suggest that this is the most likely origin for 8x L. arundinaceum. Similarly, 10x L. arundinaceum was probably the result of hybridizations involving F. mairei, 4x L. arundinaceum, and an unknown third species, or more likely between 6x L. arundinaceum and F. mairei.
Thus, as Borrill (1972) pointed out, the genesis of tetraploids and hybridization between them seem to be central factors in the evolution of species in subg. Schedonorus. It is interesting to note that 4x L. arundinaceum seemingly is confined to Europe, as F. mairei is to North Africa. How then did these geographically separated species cross-hybridize to produce the polyploids we see today? The answer may lie in desertification, believed to have drastically altered the North African landscape beginning approximately 10,000 yr ago. Before then, this region was much cooler and wetter, so these tetraploids might have shared a much larger geographic range (Chandrasekharan et al., 1972). In addition, it should be noted that 4x L. arundinaceum and L. pratense are reported to have been collected in North Africa, albeit very rarely (Saint-Yves, 1922; Maire, 1955). Thus, it is not unreasonable to propose that such natural hybridizations could have taken place before, or at an early stage in the desertification of this region.
It is worth noting that L. pratense and 6x L. arundinaceum are both infected with seed-transmitted fungal endophytes in the genus Neotyphodium. These fungi are mutualists (see Chapter 14) and capable of defending their host grasses from insect and mammalian herbivores as well as a variety of abiotic stresses (see Chapter 4, Chapter 16, and Chapter 17). Neotyphodium uncinatum and N. coenophialum (infecting L. pratense and 6x L. arundinaceum, respectively) are both hybrid fungi. Thus, the tall fescue-N. coenophialum mutualism represents one of the most complex genetic entities known in biology, with both host and symbiont having hybrid natures. More generally, hybrid endophytes are fairly common among the cool-season grasses (subfamily Pooideae). Whether there is some correlation between hybridization in the grasses and similar processes in their associated symbionts is still an open question. However, the propensity of both to generate genetic variation through hybridization may indeed promote rapid adaptability and niche occupation in the grasses.
In summary, early hybridization events between diploids such as L. pratense and F. scariosa (and perhaps others) could have given rise to the 4x species, such as F. mairei and 4x L. arundinaceum. Such tetraploids could then hybridize with each other or with other species to generate higher polyploids. If this scenario is correct, the most recent grasses are those with the highest chromosome numbers, such as 8x and 10x L. arundinaceum, which have been very successful in the Atlas Mountains of North Africa (Borrill, 1972).
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|Fig. 2-11. Lolium pratense is a pivotal diploid species believed to have contributed genomes to 4x and 6x tall fescue.|
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