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RESEARCH ARTICLE (Open Access)

Genetic parameter estimates for male and female fertility traits using genomic data to improve fertility in Australian beef cattle

Babatunde S. Olasege https://orcid.org/0000-0002-4941-9991 A B , Muhammad S. Tahir A B , Gabriela C. Gouveia C , Jagish Kour A , Laercio R. Porto-Neto B , Ben J. Hayes D and Marina R. S. Fortes https://orcid.org/0000-0002-7254-1960 A E
+ Author Affiliations
- Author Affiliations

A School of Chemistry and Molecular Biosciences, The University of Queensland, Saint Lucia Campus, Brisbane, Qld 4072, Australia.

B CSIRO Agriculture and Food, Saint Lucia, Qld 4067, Australia.

C Animal Science Department, Veterinary School, Federal University of Minas Gerais, Belo Horizonte 31270-901, Brazil.

D Queensland Alliance for Agriculture and Food Innovation (QAAFI), The University of Queensland, Saint Lucia Campus, Brisbane, Qld 4072, Australia.

E Corresponding author. Email: m.fortes@uq.edu.au

Animal Production Science - https://doi.org/10.1071/AN21097
Submitted: 22 February 2021  Accepted: 1 June 2021   Published online: 3 August 2021

Journal Compilation © CSIRO 2021 Open Access CC BY-NC-ND

Abstract

Context: Studies have shown that favourable genetic correlations exist between female and male fertility traits. However, investigations regarding these correlations in Australian tropical beef cattle are limited to either pedigree or single-breed analysis.

Aim: The study aims to use genomic information to estimate genetic parameters of six female and seven male fertility traits measured during the first 2 years of life, in two tropical breeds.

Methods: Single-, bivariate and multi-trait models were used to analyse fertility data from Brahman (BB; 996 cows and 1022 bulls); and Tropical Composite (TC; 1091 cows and 998 bulls) cattle genotyped with high-density single-nucleotide polymorphism chip assay.

Key results: Heritability estimates in BB cows ranged from low (0.07 ± 0.04) for days to calving at the first calving opportunity (DC1, days) to high (0.57 ± 0.08) for age at first corpus luteum (AGECL, days). In BB bulls, estimates varied from low (0.09 ± 0.05) for sperm motility (score 1–5) to high (0.64 ± 0.06) for scrotal circumference (SC) measured at 24 months (SC24, cm). Similarly, heritability estimates in TC cows were low (0.04 ± 0.03) for DC1 and high (0.69 ± 0.02) for AGECL. In TC bulls, the heritability was low (0.09 ± 0.05) for sperm motility and high (0.69 ± 0.07) for SC24. Within-sex for both breeds, blood concentrations of insulin growth-factor 1 (IGF1) measured in cows at 18 months (IGF1c) were negatively correlated with female fertility phenotypes. In BB, across-sex, bulls’ blood concentration of IGF1 measured at 6 months (IGF1b) was a good indicator trait for the following four female traits: AGECL, the first postpartum anoestrus interval, age at first calving and DC1. In TC, IGF1b and percentage normal sperm were good predictors of female fertility phenotypes.

Conclusions: The heritability estimates and genomic correlations from the present study generally support and confirmed the earlier estimates from pedigree analyses. The findings suggest that selection for female fertility traits will benefit male fertility, and vice versa.

Implications: Heritability estimates and genomic correlations suggest that we can select for fertility traits measured early in life, with benefits within and across sex. Using traits available through veterinary assessment of bull fertility as selection indicators will enhance bull and cow fertility, which can lead to better breeding rates in tropical herds.

Keywords: genetic correlation, fertility, heritability, beef cattle, genomics.


References

Abribat T, Lapierre H, Dubreuil P, Pelletier G, Gaudreau P, Brazeau P, Petitclerc D (1990) Insulin-like growth factor-I concentration in Holstein female cattle: variations with age, stage of lactation and growth hormone-releasing factor administration. Domestic Animal Endocrinology 7, 93–102.
Insulin-like growth factor-I concentration in Holstein female cattle: variations with age, stage of lactation and growth hormone-releasing factor administration.Crossref | GoogleScholarGoogle Scholar | 2107053PubMed |

Alves BR, Cardoso RC, Doan R, Zhang Y, Dindot SV, Williams GL, Amstalden M (2017) Nutritional programming of accelerated puberty in heifers: alterations in DNA methylation in the arcuate nucleus. Biology of Reproduction 96, 174–184.

Analla M, Munoz‐Serrano A, Cruz J, Serradilla J (1995) Estimation of genetic parameters of growth traits in Segureña lambs. Journal of Animal Breeding and Genetics 112, 183–190.
Estimation of genetic parameters of growth traits in Segureña lambs.Crossref | GoogleScholarGoogle Scholar |

Barwick S, Johnston D, Burrow H, Holroyd R, Fordyce G, Wolcott ML, Sim WD, Sullivan M (2009) Erratum to: Genetics of heifer performance in ‘wet’ and ‘dry’ seasons and their relationships with steer performance in two tropical beef genotypes. Animal Production Science 49, 727
Erratum to: Genetics of heifer performance in ‘wet’ and ‘dry’ seasons and their relationships with steer performance in two tropical beef genotypes.Crossref | GoogleScholarGoogle Scholar |

Bolormaa S, Pryce JE, Zhang Y, Reverter A, Barendse W, Hayes BJ, Goddard ME (2015) Non-additive genetic variation in growth, carcass and fertility traits of beef cattle. Genetics, Selection, Evolution 47, 26
Non-additive genetic variation in growth, carcass and fertility traits of beef cattle.Crossref | GoogleScholarGoogle Scholar | 25880217PubMed |

Brito Lopes F, da Silva MC, Magnabosco CU, Goncalves Narciso M, Sainz RD (2016) Selection indices and multivariate analysis show similar results in the evaluation of growth and carcass traits in beef cattle. PLoS One 11, e0147180
Selection indices and multivariate analysis show similar results in the evaluation of growth and carcass traits in beef cattle.Crossref | GoogleScholarGoogle Scholar | 26789008PubMed |

Burns B, Corbet N, Corbet D, Crisp J, Venus B, Johnston D, Li Y, McGowan M, Holroyd R (2013) Male traits and herd reproductive capability in tropical beef cattle. 1. Experimental design and animal measures. Animal Production Science 53, 87–100.
Male traits and herd reproductive capability in tropical beef cattle. 1. Experimental design and animal measures.Crossref | GoogleScholarGoogle Scholar |

Cole J, VanRaden P (2018) Symposium review: possibilities in an age of genomics: the future of selection indices. Journal of Dairy Science 101, 3686–3701.
Symposium review: possibilities in an age of genomics: the future of selection indices.Crossref | GoogleScholarGoogle Scholar | 29103719PubMed |

Corbet N, Burns B, Corbet D, Johnston D, Crisp J, McGowan M, Prayaga K, Venus B, Holroyd R (2009). Genetic variation in growth, hormonal and seminal traits of young tropically adapted bulls. In ‘Proceedings of the of the 18th Conference of the Association for the Advancement of Animal Breeding and Genetics (AABG)’, Barossa Valley, SA, Australia, 28 September–1 October 2009. pp. 121–124. (Association for the Advancement of Animal Breeding and Genetics)

Corbet N, Burns B, Johnston D, Wolcott ML, Corbet D, Venus B, Li Y, McGowan M, Holroyd R (2013) Male traits and herd reproductive capability in tropical beef cattle. 2. Genetic parameters of bull traits. Animal Production Science 53, 101–113.
Male traits and herd reproductive capability in tropical beef cattle. 2. Genetic parameters of bull traits.Crossref | GoogleScholarGoogle Scholar |

Das S, Forer L, Schönherr S, Sidore C, Locke AE, Kwong A, Vrieze SI, Chew EY, Levy S, McGue M (2016) Next-generation genotype imputation service and methods. Nature Genetics 48, 1284–1287.
Next-generation genotype imputation service and methods.Crossref | GoogleScholarGoogle Scholar | 27571263PubMed |

Fairbairn DJ, Blanckenhorn WU, Székely T (2007) ‘Sex, size and gender roles: evolutionary studies of sexual size dimorphism.’ (Oxford University Press)

Falkenberg U, Haertel J, Rotter K, Iwersen M, Arndt G, Heuwieser W (2008) Relationships between the concentration of insulin-like growth factor-1 in serum in dairy cows in early lactation and reproductive performance and milk yield. Journal of Dairy Science 91, 3862–3868.
Relationships between the concentration of insulin-like growth factor-1 in serum in dairy cows in early lactation and reproductive performance and milk yield.Crossref | GoogleScholarGoogle Scholar | 18832208PubMed |

Fordyce G, Entwistle K, Norman S, Perry V, Gardiner B, Fordyce P (2006) Standardising bull breeding soundness evaluations and reporting in Australia. Theriogenology 66, 1140–1148.
Standardising bull breeding soundness evaluations and reporting in Australia.Crossref | GoogleScholarGoogle Scholar | 16620941PubMed |

Fortes MR, Porto-Neto LR, Satake N, Nguyen LT, Freitas AC, Melo TP, Scalez DCB, Hayes B, Raidan FS, Reverter A (2020) X chromosome variants are associated with male fertility traits in two bovine populations. Genetics, Selection, Evolution 52, 46
X chromosome variants are associated with male fertility traits in two bovine populations.Crossref | GoogleScholarGoogle Scholar | 32787790PubMed |

Gargantini G, Cundiff LV, Lunstra DD, Van Vleck LD (2004) Genetic relationships between male and female reproductive traits in beef cattle. Journal of Animal Science 82, 37.

Hansen GR (2006) ‘Managing bull fertility in beef cattle herds.’ (Animal Science Department, Florida Cooperative Extension Service, Institute of Food of Agricultural Sciences, University of Florida: FL, USA)

Hayes B, Goddard M (2010) Genome-wide association and genomic selection in animal breeding. Genome 53, 876–883.
Genome-wide association and genomic selection in animal breeding.Crossref | GoogleScholarGoogle Scholar | 21076503PubMed |

Hayes B, Fordyce G, Landmark S (2019a) Genomic predictions for fertility traits in tropical beef cattle from a multi-breed, crossbred and composite reference population. In ‘Proceedings of the 23rd Conference of the Association for the Advancement of Animal Breeding and Genetics (AAABG)’, Armidale, NSW, Australia, 27 October – 1 November 2019. pp. 282–285. (Association for the Advancement of Animal Breeding and Genetics)

Hayes BJ, Corbet NJ, Allen JM, Laing AR, Fordyce G, Lyons R, McGowan MR, Burns BM (2019b) Towards multi-breed genomic evaluations for female fertility of tropical beef cattle. Journal of Animal Science 97, 55–62.
Towards multi-breed genomic evaluations for female fertility of tropical beef cattle.Crossref | GoogleScholarGoogle Scholar | 30371787PubMed |

Jeyaruban M, Johnston D (2017) Genetic Association of Young Male Traits with Female Reproductive Performance in Brahman and Santa Gertrudis Cattle. In ‘Proceedings of the Association for the Advancement of Animal Breeding and Genetics (AABG)’, Townsville, QLD, Australia, 2 July–5 July 2017. pp. 305–308 (Association for the Advancement of Animal Breeding and Genetics.

Johnson T, Keehan M, Harland C, Lopdell T, Spelman R, Davis S, Rosen B, Smith T, Couldrey C (2019) Identification of the pseudoautosomal region in the Hereford bovine reference genome assembly ARS-UCD1. 2. Journal of Dairy Science 102, 3254–3258.
Identification of the pseudoautosomal region in the Hereford bovine reference genome assembly ARS-UCD1. 2.Crossref | GoogleScholarGoogle Scholar | 30712931PubMed |

Johnston D, Moore K (2019) Genetic Correlations Between Days to Calving and Other Male and Female Reproduction Traits in Brahman Cattle. In ‘Proceedings of the Association for the Advancement of Animal Breeding and Genetics’, pp. 354–357.

Johnston D, Barwick S, Corbet N, Fordyce G, Holroyd R, Williams PJ, Burrow HM (2009) Genetics of heifer puberty in two tropical beef genotypes in northern Australia and associations with heifer-and steer-production traits. Animal Production Science 49, 399–412.
Genetics of heifer puberty in two tropical beef genotypes in northern Australia and associations with heifer-and steer-production traits.Crossref | GoogleScholarGoogle Scholar |

Johnston D, Barwick S, Fordyce G, Holroyd R, Williams P, Corbet N, Grant T (2014a) Genetics of early and lifetime annual reproductive performance in cows of two tropical beef genotypes in northern Australia. Animal Production Science 54, 1–15.
Genetics of early and lifetime annual reproductive performance in cows of two tropical beef genotypes in northern Australia.Crossref | GoogleScholarGoogle Scholar |

Johnston D, Corbet N, Barwick S, Wolcott ML, Holroyd R (2014b) Genetic correlations of young bull reproductive traits and heifer puberty traits with female reproductive performance in two tropical beef genotypes in northern Australia. Animal Production Science 54, 74–84.
Genetic correlations of young bull reproductive traits and heifer puberty traits with female reproductive performance in two tropical beef genotypes in northern Australia.Crossref | GoogleScholarGoogle Scholar |

Land R (1973) The expression of female sex-limited characters in the male. Nature 241, 208–209.
The expression of female sex-limited characters in the male.Crossref | GoogleScholarGoogle Scholar | 4700891PubMed |

Lande R (1984) The genetic correlation between characters maintained by selection, linkage and inbreeding. Genetical Research 44, 309–320.
The genetic correlation between characters maintained by selection, linkage and inbreeding.Crossref | GoogleScholarGoogle Scholar | 6530140PubMed |

Larnkjær A, Mølgaard C, Michaelsen KF (2012) Early nutrition impact on the insulin-like growth factor axis and later health consequences. Current Opinion in Clinical Nutrition and Metabolic Care 15, 285–292.
Early nutrition impact on the insulin-like growth factor axis and later health consequences.Crossref | GoogleScholarGoogle Scholar | 22466924PubMed |

Lesmeister J, Burfening P, Blackwell R (1973) Date of first calving in beef cows and subsequent calf production. Journal of Animal Science 36, 1–6.
Date of first calving in beef cows and subsequent calf production.Crossref | GoogleScholarGoogle Scholar |

Loh P-R, Danecek P, Palamara PF, Fuchsberger C, Reshef YA, Finucane HK, Schoenherr S, Forer L, McCarthy S, Abecasis GR (2016) Reference-based phasing using the Haplotype Reference Consortium panel. Nature Genetics 48, 1443
Reference-based phasing using the Haplotype Reference Consortium panel.Crossref | GoogleScholarGoogle Scholar | 27694958PubMed |

Lopez R, Thomas M, Hallford D, Keisler D, Silver G, Obeidat B, Garcia M, Krehbiel C (2006) Metabolic hormone profiles and evaluation of associations of metabolic hormones with body fat and reproductive characteristics of Angus, Brangus, and Brahman heifers. The Professional Animal Scientist 22, 273–282.
Metabolic hormone profiles and evaluation of associations of metabolic hormones with body fat and reproductive characteristics of Angus, Brangus, and Brahman heifers.Crossref | GoogleScholarGoogle Scholar |

Lunstra D, Cundiff L (2003) Growth and pubertal development in Brahman-, Boran-, Tuli-, Belgian blue-, Hereford- and Angus-sired f1 bulls. Journal of Animal Science 81, 1414–1426.
Growth and pubertal development in Brahman-, Boran-, Tuli-, Belgian blue-, Hereford- and Angus-sired f1 bulls.Crossref | GoogleScholarGoogle Scholar | 12817488PubMed |

Lunstra D, Ford J, Echternkamp S (1978) Puberty in beef bulls: hormone concentrations, growth, testicular development, sperm production and sexual aggressiveness in bulls of different breeds. Journal of Animal Science 46, 1054–1062.
Puberty in beef bulls: hormone concentrations, growth, testicular development, sperm production and sexual aggressiveness in bulls of different breeds.Crossref | GoogleScholarGoogle Scholar | 566747PubMed |

Martinez-Velazquez G, Gregory K, Bennett G, Van Vleck LD (2003) Genetic relationships between scrotal circumference and female reproductive traits. Journal of Animal Science 81, 395–401.
Genetic relationships between scrotal circumference and female reproductive traits.Crossref | GoogleScholarGoogle Scholar | 12643482PubMed |

Matukumalli LK, Lawley CT, Schnabel RD, Taylor JF, Allan MF, Heaton MP, O’Connell J, Moore SS, Smith TP, Sonstegard TS (2009) Development and characterization of a high density SNP genotyping assay for cattle. PLoS One 4, e5350
Development and characterization of a high density SNP genotyping assay for cattle.Crossref | GoogleScholarGoogle Scholar | 19390634PubMed |

Michaelsen KF (2013) Effect of protein intake from 6 to 24 months on insulin-like growth factor 1 (IGF-1) levels, body composition, linear growth velocity, and linear growth acceleration: what are the implications for stunting and wasting? Food and Nutrition Bulletin 34, 268–271.
Effect of protein intake from 6 to 24 months on insulin-like growth factor 1 (IGF-1) levels, body composition, linear growth velocity, and linear growth acceleration: what are the implications for stunting and wasting?Crossref | GoogleScholarGoogle Scholar | 23964408PubMed |

Misztal I, Tsuruta S, Strabel T, Auvray B, Druet T, Lee D (2002) BLUPF90 and related programs (BGF90). In ‘Proceedings of the 7th world congress on genetics applied to livestock production’, pp. 743–744.

Moore L, Pfeffer A, Chie WN, Miller H, Rogers K, O’Keeffe L (1995) Induction of an acute phase response in lambs causes an increase in plasma levels of GH and IGF-I. The Journal of Endocrinology 144, 243–250.
Induction of an acute phase response in lambs causes an increase in plasma levels of GH and IGF-I.Crossref | GoogleScholarGoogle Scholar | 7706978PubMed |

Pennell TM, Morrow EH (2013) Two sexes, one genome: the evolutionary dynamics of intralocus sexual conflict. Ecology and Evolution 3, 1819–1834.
Two sexes, one genome: the evolutionary dynamics of intralocus sexual conflict.Crossref | GoogleScholarGoogle Scholar | 23789088PubMed |

Poissant J, Wilson AJ, Coltman DW (2010) Sex‐specific genetic variance and the evolution of sexual dimorphism: a systematic review of cross‐sex genetic correlations. Evolution 64, 97–107.
Sex‐specific genetic variance and the evolution of sexual dimorphism: a systematic review of cross‐sex genetic correlations.Crossref | GoogleScholarGoogle Scholar | 19659596PubMed |

Porto-Neto L, Lehnert S, Fortes M, Kelly M, Reverter A (2013a) Population stratification and breed composition of Australian tropically adapted cattle. In ‘Proceedings of the Twentieth Conference of the Association for the Advancement of Animal Breeding and Genetics’, pp. 147–150.

Porto-Neto LR, Sonstegard TS, Liu GE, Bickhart DM, Da Silva MV, Machado MA, Utsunomiya YT, Garcia JF, Gondro C, Van Tassell CP (2013b) Genomic divergence of zebu and taurine cattle identified through high-density SNP genotyping. BMC Genomics 14, 876
Genomic divergence of zebu and taurine cattle identified through high-density SNP genotyping.Crossref | GoogleScholarGoogle Scholar | 24330634PubMed |

Raidan FS, Porto-Neto LR, Reverter A (2019) Across-sex genomic-assisted genetic correlations for sex-influenced traits in Brahman cattle. Genetics, Selection, Evolution 51, 41
Across-sex genomic-assisted genetic correlations for sex-influenced traits in Brahman cattle.Crossref | GoogleScholarGoogle Scholar | 31337334PubMed |

Rasali D, Shrestha J, Crow G (2006) Development of composite sheep breeds in the world: a review. Canadian Journal of Animal Science 86, 1–24.

Rosen BD, Bickhart DM, Schnabel RD, Koren S, Elsik CG, Tseng E, Rowan TN, Low WY, Zimin A, Couldrey C (2020) De novo assembly of the cattle reference genome with single-molecule sequencing. GigaScience 9, giaa021
De novo assembly of the cattle reference genome with single-molecule sequencing.Crossref | GoogleScholarGoogle Scholar | 32543654PubMed |

Schatz T, Jayawardhana G, Golding R, Hearnden M (2010) Selection for fertility traits in Brahmans increases heifer pregnancy rates from yearling mating. Animal Production Science 50, 345–348.
Selection for fertility traits in Brahmans increases heifer pregnancy rates from yearling mating.Crossref | GoogleScholarGoogle Scholar |

Taylor JF (2014) Implementation and accuracy of genomic selection. Aquaculture 420, S8–S14.
Implementation and accuracy of genomic selection.Crossref | GoogleScholarGoogle Scholar |

Taylor V, Cheng Z, Pushpakumara P, Wathes D, Beever D (2004) Relationships between the plasma concentrations of insulin-like growth factor-I in dairy cows and their fertility and milk yield. The Veterinary Record 155, 583–588.
Relationships between the plasma concentrations of insulin-like growth factor-I in dairy cows and their fertility and milk yield.Crossref | GoogleScholarGoogle Scholar | 15573950PubMed |

Tiezzi F, Maltecca C (2011) Selecting for female fertility: what can be learned from the dairy experience. Beef Improvement Federation, Research Symposium & Annual Meeting. Montana, USA, pp. 47–60.

VanRaden PM (2008) Efficient methods to compute genomic predictions. Journal of Dairy Science 91, 4414–4423.
Efficient methods to compute genomic predictions.Crossref | GoogleScholarGoogle Scholar | 18946147PubMed |

Velazquez M, Spicer L, Wathes D (2008) The role of endocrine insulin-like growth factor-I (IGF-I) in female bovine reproduction. Domestic Animal Endocrinology 35, 325–342.
The role of endocrine insulin-like growth factor-I (IGF-I) in female bovine reproduction.Crossref | GoogleScholarGoogle Scholar | 18703307PubMed |

Weigel KA (2017) Genomic selection of dairy cattle: a review of methods, strategies, and impact. Journal of Animal Breeding and Genetics 1, 1–15.

Wellmann R, Bennewitz J (2019) Key Genetic Parameters for population management. Frontiers in Genetics 10, 667
Key Genetic Parameters for population management.Crossref | GoogleScholarGoogle Scholar | 31475027PubMed |

Wolcott ML, Johnston D, Barwick S, Corbet N, Burrow HM (2014) Genetic relationships between steer performance and female reproduction and possible impacts on whole herd productivity in two tropical beef genotypes. Animal Production Science 54, 85–96.
Genetic relationships between steer performance and female reproduction and possible impacts on whole herd productivity in two tropical beef genotypes.Crossref | GoogleScholarGoogle Scholar |

Yang J, Lee SH, Goddard ME, Visscher PM (2011) GCTA: a tool for genome-wide complex trait analysis. American Journal of Human Genetics 88, 76–82.
GCTA: a tool for genome-wide complex trait analysis.Crossref | GoogleScholarGoogle Scholar | 21167468PubMed |

Yilmaz A, Davis M, Simmen R (2004) Estimation of (co) variance components for reproductive traits in Angus beef cattle divergently selected for blood serum IGF-I concentration. Journal of Animal Science 82, 2285–2292.
Estimation of (co) variance components for reproductive traits in Angus beef cattle divergently selected for blood serum IGF-I concentration.Crossref | GoogleScholarGoogle Scholar | 15318726PubMed |

Yilmaz A, Davis M, Simmen R (2006) Analysis of female reproductive traits in Angus beef cattle divergently selected for blood serum insulin-like growth factor I concentration. Theriogenology 65, 1180–1190.
Analysis of female reproductive traits in Angus beef cattle divergently selected for blood serum insulin-like growth factor I concentration.Crossref | GoogleScholarGoogle Scholar | 16144706PubMed |

Zhang Y, Johnston D, Bolormaa S, Hawken R, Tier B (2014) Genomic selection for female reproduction in Australian tropically adapted beef cattle. Animal Production Science 54, 16–24.
Genomic selection for female reproduction in Australian tropically adapted beef cattle.Crossref | GoogleScholarGoogle Scholar |

Zhang X, Tsuruta S, Andonov S, Lourenco D, Sapp R, Wang C, Misztal I (2018) Relationships among mortality, performance, and disorder traits in broiler chickens: a genetic and genomic approach. Poultry Science 97, 1511–1518.
Relationships among mortality, performance, and disorder traits in broiler chickens: a genetic and genomic approach.Crossref | GoogleScholarGoogle Scholar | 29529319PubMed |

Zulu VC, Nakao T, Sawamukai Y (2002) Insulin-like growth factor-I as a possible hormonal mediator of nutritional regulation of reproduction in cattle. The Journal of Veterinary Medical Science 64, 657–665.
Insulin-like growth factor-I as a possible hormonal mediator of nutritional regulation of reproduction in cattle.Crossref | GoogleScholarGoogle Scholar | 12237508PubMed |