Tertiary trisomics in the garden pea as a model of B chromosome evolution in plants

Abstract

It is hypothesized that, in plants, genetically empty B chromosomes may originate from the extra chromosome (E) of tertiary trisomics if (i) the region of basic chromosomes homologous to the E (H-region) harbors a sporophytic lethal covered by the wild-type allele in E, and (ii) crossing-over between E and the H-region is suppressed. Under these conditions, most loss-of-function mutations occurring in the H-region are deleterious for haploid gametophytes, whereas those occurring in E are neutral or advantageous for hyperploid (n þ 1) gametophytes. As a result, natural selection at the gametophyte level can lead to the degeneration of E, leaving the H-region intact. Using Hammarlund translocation T(3-6)a, we synthesized two trisomic lines of the garden pea (Pisum sativum L.), where E was composed of the short arms of chromosomes 3 and 6 and the H-region carried recessive markers. In the trisomic line TRIS, we found few crossovers between E and the H-region. In the trisomic line TRUST, obtained after a change of basic chromosome constitution, recombination in this region was completely suppressed. After induction in the H-region of TRUST of a recessive sporophytic mutation rmv, two 15-chromosome lines of stable trisomics were established. One of them passed 11 generations, having produced more than 6000 individuals, all of them trisomic, and E remained present as a single element with no pairing partners. No tetrasomics were dete-cted in these lines. If such trisomics occurred in nature, their extra chromosomes are likely to become a B chromosome.

Sequences associated with A chromosome centromeres are present throughout the maize B chromosome

“…Because B chromosomes do not confer advantages to organisms that harbor them, they may be thought of as parasitic chromosomes that persist in populations by making use of cellular machinery required for normal chromosome maintenance and passage (reviewed in Puertas 2002;Jones and Houben 2003). Therefore, the means by which B chromosomes arise and the mechanisms that allow them to persist have been studied in an effort to understand how regular chromosomes are maintained (e.g., Berdnikov et al 2003;Cheng et al 2000;Dhar et al 2002;Kaszas and Birchler 1998).…”

Chromosomes with a life of their own

“…Nobody really knows where B chromosomes come from, other than to say that they originate from the As, although there are some plausible ideas and models (Jamilena et al, 1994;Dhar et al, 2002;Berdnikov et al, 2003;Jones and Houben, 2003).…”

Common Descent of B Chromosomes in Two Species of the Fish Genus <b><i>Prochilodus</i></b> (Characiformes, Prochilodontidae)

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hypothesis predicts that B chromosomes are old genomic elements in this genus, and this could be tested once a species from a relative genus would be found showing homology of its A chromosomes with the B-probes employed here, through a comparison of B chromosome DNA sequences with those in the A chromosomes of this other species.

© 2013 S. Karger AG, BaselMany particular hypotheses attempt to explain the possible ways by which B chromosomes can be generated among distinct organisms [Jones, 1995;Camacho et al, 2000;Berdnikov et al, 2003;Jones and Houben, 2003]. However, although these chromosomes may have different origins, 2 general hypotheses differ in whether B chromosomes originated intraspecifically from the members of the standard (A) chromosome set [Jamilena et al, 1994[Jamilena et al, , 1995Houben et al, 1996;Peppers et al, 1997;Cabrero et al, 1999 ] or interspecifically through hybridization between closely related species yielding the B chromosome as a byproduct [Sapre and Deshpande, 1987;Eickbush et al, 1992;McVean, 1995;Perfectti and Werren, 2001].

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