Compensated aneuploidy

Lim et al. (2008) used genomic and fluorescent in situ hybridization (GISH/FISH) in a preliminary survey of several T. mirus and T. miscellus plants and found a few plants of each species that were not chromosomally additive of the diploid parents. Several natural T. mirus and T. miscellus plants were found to be aneuploid as well as showing intergenomic translocations. Although most of the aneuploid individuals examined were 2n = 24, not all chromosomes were present in two copies as expected.

The preliminary work of Lim et al. (2008) prompted a detailed examination using GISH and FISH of the chromosomal variation generated in six T. miscellus populations from the Palouse (Chester et al., 2012). These populations represent independently derived lineages that at the time of sampling were approximately 40 generations old for this biennial. In all six populations both aneuploidy and translocations were common (Fig. 5). Only three of the 58 plants exhibited the expected additivity of the diploid parental karyotypes (i.e., showed neither aneuploidy nor translocations). Although approximately 69% of polyploid plants were aneuploid for one or more chromosomes, variation in copy number appears to be constrained. Most plants were 2n = 24 and the total copy number for each homeologous group of chromosomes was typically four because of aneuploidy being reciprocal between homeologous chromosomes (see Figure). Thus, most deviations from disomy were in the form of monosomy-trisomy or nullisomy-tetrasomy, between homeologous chromosomes. This pattern of extensive aneuploidy while maintaining the overall copy number closely resembles cytological changes in synthetic neoallotetraploid Brassica napus (Xiong et al. 2011). Gene dosage was implicated as a major factor constraining chromosomal changes such that imbalances, which arise, require compensation by chromosomes (compensatory aneuploidy) or homeologous segments (compensatory translocations).

A similar detailed cytogenetic study of T. mirus followed (Chester et al. 2015) also revealed compensated aneuploidy.  Of 37 T. mirus individuals that were karyotyped, 23 (62%) were chromosomally additive of the parents, whereas the remaining 14 individuals (38%) had aneuploid compositions. The proportion of additive versus aneuploid individuals differed from that found previously in T. miscellus, in which aneuploidy was more common (69%).

Application of GISH to synthetic lines of both allotetraploids revealed that  aneuploidy developed quickly in synthetics and resembled that of naturally occurring T. mirus and T. miscellus by generation S₄ (Spoelhof et al., 2017).

The observed chromosomal changes in the recently formed Tragopogon allotetraploids provide two possible explanations for the loss of DNA that has been observed using molecular SNP-based assays.  (1) If a translocation was in a non-reciprocal state and homozygous, this could lead to the loss of DNA from one of the parental diploids in the translocated region. (2) Nullisomy would lead to the complete loss of DNA from one of the parental chromosomes. However, the diverse patterns of homeolog losses that have been detected are not readily explainable only by these large-scale chromosomal changes detectable with GISH.