Principios de Taxonomia

6.26 Hybridogenic Speciationj175
of the genomes during the formation of the haploid germ cells does not occur.
Sperm and eggs in the F1 hybrid consequently do not receive complete genetic
sets and thus are not fully equipped with all the necessary genes. Alternatively,
they receive supernumerary genes or chromosomes, which also may cause
sterility. This is one reason why there are postzygotic reproductive barriers
between different species. This is also a reason why, in species crossings, the
fertility of the species hybrid is affected rather than its vitality. A species hybrid has
a higher chance of reaching adulthood and thriving than of actually being fertile
(Wu, Johnson, and Palopoli, 1998).
However, it is remarkable that this problem of the disturbed meiotic tetrad
formation in species hybrids occurs significantly less often in plants than in
animals. There is a particular reason for this. Plants can (for mostly unknown
reasons) more easily live in a tetraploid or higher polyploid state than animals. If
the plant deviates from the norm and has four or even more chromosome sets in
each cell instead of the usual two (diploid), this frequently has no apparent
consequences. This is a great contrast from animals. Rough estimates state that
one-third of all plant species have a polyploid origin (Schilthuizen, 2001). Many
cultivated plants, for example, are tetraploid. If the supplementary chromosome
sets stem from the same species, this is called autopolyploidy; if they stem from
different species, then this is called allopolyploidy.
In the crossing of two different species, the zygote of the new species hybrid
contains two different genomes, which stem from the two parental species: one
chromosome set is obtained from the father species, and the other chromosome
set is obtained from the mother species. The hybrid is therefore allodiploid. This
can, and in fact, usually does lead to disturbances in the meiotic tetrad formation.
However, plants can easily become tetraploid and they can bypass the upcoming
meiotic disturbances because when every chromosome finds a conspecific
homologous partner, then allotetraploid meiotic cells can form entirely normal
tetrads. The only difference compared to the meiosis of the purebred parents is
that the number of tetrads has doubled in allotetraploid organisms; the tetrads
themselves are just like those in the purebred diploid parents.
If such allotetraploid cells in the hybrid organism undergo meiosis and
reductional divisions, the resulting gametes are not haploid; instead, they are
diploid. Diploidy of the germ cells appears not to block the function of the mature
germ cells; thus, allotetraploid hybrid organisms can produce zygotes and vital
tetraploid offspring.
Thus, tetraploidy explains why species hybrids in plants are frequently fertile in
producing viable germ cells. This still does not explain why hybridogenic
speciation is possible in plants. Hybridogenic speciation requires the existence
of a barrier against backcrossing with the parental species.
At the same time, tetraploidy also explains this barrier. If a diploid mature germ
cell of the hybrid fuses with a haploid germ cell of the parental species through
backcrossing, then a triploid zygote is generated, and the offspring of this zygote
would be triploid. A triploid organism, however, would then be incapable of
meiosis because no appropriate tetrad formation would be possible in triploid