Cross Incompatibility: A Barrier to Variation

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Cross-incompatibility

Cross incompatibility (CI) has been defined as any relationship (or its absence) between pollen and pistil which prevents hybrid zygote formation in crosses between two fertile species (de Nettancourt 2001).

CI can be either unilateral or bilateral, depending on whether a given genotypic combination is, respectively, compatible in only one direction or in both directions of the cross.

  • Like if A*B is possible but B*A is not possible, it is unidirectional.
  • If both are possible, it is bidirectional.

Arrest of pollen tube growth in CI can occur at various sites of the pistil (stigma; upper, middle or bottom part of the style) or in the ovary, as observed in various members of the Solanaceae family: wild potatoes (Camadro and Peloquin 1981; Hayes et al. 2005), wild tomatoes (Baek et al. 2015) and pepper (Onus and Pickersgill 2004).

Before delving into Cross Incompatibility, let’s look on to some vectors of pollination.

 

Agents of  Pollination

Types

  •  Pre-zygotic: Occurs before zygote formation
  •  Post-zygotic: Occurs after zygote formation

Significance: Plant speciation is characterized by the evolution of barriers to genetic exchange between previously interbreeding populations.

Pre-zygotic barrier

Anything that impedes pollination and fertilization is a pre-zygotic barrier. It typically contributes more to total reproductive isolation in plants than do post-zygotic barriers.

Examples include:

  • Habitat isolation
  • Behavioral isolation
  • Temporal isolation
  • Mechanical isolation
  • Gametic isolation

Post-zygotic barrier

Post-zygotic barriers prevent a hybrid zygote from developing into a viable, fertile adult plant.

Reduced viability or fertility of hybrid embryos or individuals are evidence of postzygotic reproductive isolation. Differences in chromosome number or arrangement of genes on chromosomes usually result in postzygotic isolation because chromosomes may not pair normally during mitosis or meiosis.

Reasons for Cross-incompatibility

  • Due to failure of embryo formation.
  • Due to interaction between embryo and endosperm.
  • Formation of empty seeds.

How to overcome cross-incompatibility

  • Ovule/embryo culture
  • In vitro pollination/hybridization
  • Somatic hybridization

Ovule or embryo culture: It is a laboratory technique in which the ovules or embryos are rescued from the flower aspetically and then grown in the culture medium in a controlled condition. This culture is a boon for the plant breeders in obtaining seedlings from crosses which are normally unsuccessful because of abortive embryos.

In vitro pollination: The process of seed formation through pollination of cultured pistil is called in vitro pollination.

Somatic hybridization: The cellular genomes are mixed by protoplast fusion of two different species make a hybrid called somatic hybrid or cybrid.

 


References:

Canadian Science Publishing (Journal on Botany)

https://www.merriam-webster.com/

Class Lectures of Rita Sarah Borna, Assistant Professor, Department of Botany, University of Dhaka