Rapid Multi-Hybridisation FISH Screening for Balanced Porcine Reciprocal Translocations Suggests a Much Higher Abnormality Rate Than Previously Appreciated

Abstract

With demand rising, pigs are the world’s leading source of meat protein; however significant economic loss and environmental damage can be incurred if boars used for artificial insemination (AI) are hypoprolific (sub-fertile). Growing evidence suggests that semen analysis is an unreliable tool for diagnosing hypoprolificacy, with litter size and farrowing rate being more applicable. Once such data are available, however, any affected boar will have been in service for some time, with significant financial and environmental losses incurred. Reciprocal translocations (RTs) are the leading cause of porcine hypoprolificacy, reportedly present in 0.47% of AI boars. Traditional standard karyotyping, however, relies on animal specific expertise and does not detect more subtle (cryptic) translocations. Previously, we reported development of a multiple hybridisation fluorescence in situ hybridisation (FISH) strategy; here, we report on its use in 1641 AI boars. A total of 15 different RTs were identified in 69 boars, with four further animals XX/XY chimeric. Therefore, 4.5% had a chromosome abnormality (4.2% with an RT), a 0.88% incidence. Revisiting cases with both karyotype and FISH information, we reanalysed captured images, asking whether the translocation was detectable by karyotyping alone. The results suggest that chromosome translocations in boars may be significantly under-reported, thereby highlighting the need for pre-emptive screening by this method before a boar enters a breeding programme.

Measurement of Chromosomal Arms and FISH Reveal Complex Genome Architecture and Standardized Karyotype of Model Fish, Genus Carassius

“…Knowledge of the karyotype is necessary in (cyto)genetics, or related fields, in order to study chromosomal rearrangements and abnormalities [5,6], the identification of sex chromosomes [7,8], and/or chromosome-specific genes [9]. Although chromosomal changes, (i.e., chromosomal losses, duplications, rearrangements), are strictly linked to a certain locus, without the knowledge of detailed karyotype, it is not possible to precisely identify chromosomal aberration, mutation or syndrome [10,11].…”

Incidence, Reproductive Outcome, and Economic Impact of Reciprocal Translocations in the Domestic Pig

“…A major biological issue that causes subfertility in mammals is that of chromosomal abnormalities (mostly balanced reciprocal translocations-RTs), which are relatively common and a primary cause of reduction in litter sizes (hypoprolificacy) in boars [6][7][8]. RTs of likely de novo origin have been discovered in roughly 1:200 AI boars [9], while a report screening related individuals discovered a prevalence for these errors as high as 1:30 [10].…”

“…Around half of all boars exhibiting hypoprolificacy are the carriers of an RT, despite displaying a normal phenotype and good semen parameters [12]. Carriers of balanced RTs suffer significant reductions in farrowing rates and litter size because these chromosomal abnormalities produce up to 50% unbalanced gametes during meiosis [7,9,12,13]. As such, RTs are heritable and persist in roughly half of the surviving offspring of the carrier.…”

“…Karyotyping is widely employed as a screening method for chromosomal abnormalities, but the resolution of this technique is limited, and its accuracy is operator dependent. A potential solution is a novel device established by our group as discussed in O'Connor and colleagues [15], where we reported the development of a new screening protocol using fluorescence in situ hybridisation (FISH) based on multiple hybridization of sub-telomeric probes [7,15]. FISH-based screening is better suited than karyotyping for detecting small (cryptic) translocations and offers an output less subject to the skill of the operator performing the analysis.…”

“…FISH-based screening is better suited than karyotyping for detecting small (cryptic) translocations and offers an output less subject to the skill of the operator performing the analysis. As a result, a greater proportion of RTs are detected [7,15].…”