An Apomixis-Gene’s View on Dandelions

Abstract

In asexual organisms, the clone constitutes a level above the individual. Most dandelions (Taraxacum officinale s.l.) reproduce asexually through apomixis, asexual reproduction through seeds. A clone can be seen as a superorganism that is born, that growths, degenerates and eventually dies. Apomixis in dandelions is controlled by a few dominant genes, the so called apomixis-genes. This implies that there should be three hierarchical levels in a field of dandelions: 1. the individual plant, 2. the clone and 3. the apomixis gene. Using co-dominant genetic markers that are linked to a dominant apomixis gene, we provide evidence that this hierarchical structure indeed exists in apomictic dandelion populations. The apomixis gene view implies that whereas individual clones may go extinct due to deleterious mutation accumulation or the lack of adaptive potential, apomixis genes can prevail much longer periods of evolutionary time in a succession of clones. We provide evidence that an apomixis-gene in Taraxacum is not transmitted to diploid offspring, which could explain the absence of apomixis in diploid dandelions. Haploid non-transmission may be caused by a mutation load that is linked to the apomixis genes as a consequence of the deep asexual reproduction history of these genes residing in many clones in the past.

Appendix

A pollen-sterile sexual diploid dandelion (TJX3-20) was crossed with a pollen-fertile apomictic triploid (A68) (Fig. 22.10). TJX3-20 originated from Langres (France). A68 originated from Heteren (The Netherlands). Viable seed set in the TJX3-20 X A68 cross was low (on average 2.1%), reflecting the high frequency of inviable aneuploid pollen grains produced by the unbalanced pollen meiosis of triploid A68. Sixty-two crossed capitula contained only 192 viable seeds in total. Ninety-six F₁ plants were diploid (50%), 95 triploid (49.5%) and one (0.5%) was tetraploid. These plants were the products of the fertilization of a haploid egg cell by a haploid, diploid and triploid pollen grain, respectively.

Fig. 22.10

Spontaneous seed set of bagged flowers (excluding cross pollen) in the diploid sexual male sterile TJX3-20 seed parent (left) and the triploid apomictic pollen parent A68 (right). Large seed heads indicate apomictic seed set, small seed heads indicate the absence of spontaneous seed development

To induce flowering, eight week old F₁ plants were vernalized for 9 weeks in a cold room at 4°C. One hundred eighty two F₁ plants (94.7%) were tested for the ability to form apomictic seeds (six seedlings died and four adult plants did not flower). In order to prevent contamination by cross-pollination, the flowers were covered with small paper bags before opening. All F₁ plants were male sterile, like TJX3-20, hence seed set due to selfing can be excluded. The development of a large seed head is an indication for apomictic seed set (see Fig. 22.10). To determine the degree of apomictic seed set, for each F1 plant two batches of 50 randomly chosen seeds were germinated and the number of seedlings germinating was counted. Most of the apomicticly reproducing triploid F₁ plants had a high penetrance of apomixis (> 90% seed set), some however had a much lower penetrance.

The segregation of two microsatellite loci, MSTA53 and MSTA78, which were known to be linked to the DIP-locus were analysed, was investigated using the methods described in Falque et al. (1998) and Van Dijk and Bakx-Schotman (2004). The MSTA53 and MSTA78 genotypes of TJX3-20 were respectively 202/202 and 162/166 (in base pairs). For convenience these genotypes are renamed as b/b and a/b. The MSTA53 and MSTA78 genotypes of A68 were respectively 198/202/222 and 164/170/174. For convenience these genotypes are renamed as a/b/c and a/c/d.

All 28 F₁ triploids that reproduced apomictically carried the paternal MSTA78-164 allele (χ² = 12.65; d. f. = 1; P = 0.0004), supporting the previously reported tight linkage between MSTA-78 and the Dip-locus. Twenty four of the 28 F₁ triploids that reproduced apomictically carried the paternal MSTA53-202-allele (χ² = 2.05; d.f. = 1; P = 0.15). This implies that the MSTA78-164 allele is closer to D than the MSTA53-202 allele – in the 2x X 4x cross described by Van Dijk and Bakx-Schotman (2004) no recombinants between MSTA53 and MSTA78 were found. The other paternal and maternal alleles of MSTA78 and MSTA53 were not significantly associated with apomixis in the triploid offspring, supporting the Dip-genotype constitution Ddd.

The fact that the microsatellites are codominant and that D occurs in a single dose, allowed genetic mapping of all three homologs in the diploid pollen grains of A68 (Wu et al 1992; Van Dijk and Schotman 2004). Because the genotypes in diploid pollen grains derived from a triploid segregate in a 2:1 and not in a 1:1 ratio, we balanced the data set by constructing a complementary haploid counterpart of each diploid pollen grain. The modified data set was analyzed with Joinmap® 3.0 (Van Ooijen and Voorrips 2001) using the BC1 module and the Kosambi mapping function. Figure 22.8B shows the genetic map of the D-chromosomal region, based on the diploid pollen grains.

For the linkage between the haploid pollen lethal and the MSTA78-164 allele we assumed that the number of 164-pollen grains formed was equal to the number of c and d-pollen grains, on average 48. In only one of these MSTA78-164-pollen grains there was a cross-over between the 164-allele and the recessive lethals, resulting in a Kosambi distance of 2.1 cM. Similarly a Kosambi-distance between the MSTA53-202 allele and the recessive pollen lethal was estimated as 23.3 cM. Figure 22.8C shows the genetic map, based on haploid pollen grains, assuming recessive pollen lethality completely linked to the Dip-allele.