Hi Sarvjeet,

(1) Possible reasons for hybrid breakdown:

Cyto-nuclear Genomic Interactions and Hybrid Breakdown

Annual Review of Ecology, Evolution, and Systematics

Vol. 44: 281-302 (Volume publication date November 2013)

"Reduced fitness in interpopulation hybrids can be a first indication of genetic incompatibilities that may ultimately lead to reproductive isolation and speciation. A growing number of cases of hybrid breakdown have been traced to incompatibilities between the nuclear genome and the organellar genomes of the mitochondria and chloroplasts. Although these organellar genomes derive from ancient bacterial endosymbioses, they have been vastly reduced in size and now encode relatively few genes. The remaining genes are necessary but not sufficient for organelle function. In fact, most proteins functioning in the organelles are encoded in the nuclear genome and need to be imported after synthesis in the cytosol. The necessary interactions between organelle and nuclear genomes have resulted in some degree of coadaptation within all natural populations. Hybridization brings together previously untested allelic combinations and can disrupt intergenomic coadaptation, resulting in organelle dysfunction and, consequently, hybrid breakdown."

(2) One example of hybrid breakdown from mammalian mouse (see attachment):

Research result: "This study revealed that incompatibility of interactions of X-linked gene(s) with autosomal and/or Y-linked gene(s) causes the hybrid breakdown between the genetically distant C57BL/6J and MSM/Ms strains of mice."

Thank you Yuan.

A potential cause of hybrid lethality is vitrification per a recent PLoS ONE paper, "Species Origin of Genomic Factors in Nicotiana nudicaulis Watson Controlling Hybrid Lethality in Interspecific Hybrids between N. nudicaulis Watson and N. tabacum L".

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0097004

When two interspecies cross, they obtain 'F1 hybrid'. F1 hybrid derived from them may encounter pollen sterility, or hybrid lethality etc. The occurrence or the term of hybrid breakdown usually refers to at the F2 or later generations. See attachment.

Thank you Randall and Yuan for suggesting good articles.

hybrid breakdown is largely due to incompatibility of genes at the unlinked epistatic loci.