Modern Ross308 broilers purchased in 2019 at 50 d age and raised on an industry relevant ad libitum diet to 64 d possess a breast muscle mitochondrial content of 2.05% (Table 2). This is half that of broilers phenotyped at 4.1% in the late 1970’s (6, 7). This indicates that 45 years of additional genetic selection on muscle mass and feed efficiency has further reduced muscle aerobic capacity from what were already very low levels. It is possible some of the observed reduction is environmental in origin (e.g. diet, thermal regimen or activity levels). However, a partitioning analysis based on reciprocally feeding 1957 and 2001 broilers 1957 and 2001 diets concluded that numerous improvements in broiler performance are 85–90% attributable to genetic gain (8, 9). Moreover, given these birds were free-ranging prior to purchase at d 50 their muscle mitochondrial content would have, if anything, been stimulated by exercise compared to more intensively reared, more rapidly grown, heavier (~ 4 kg) industry birds. As such the low 2.05% estimate we have produced for these ~ 3 kg birds can be considered a conservative estimate. The true value for industry birds is likely even lower.
This combination of data from both TEM and qRT-PCR technologies taken from similar tissue samples now allows us to reassess the likely mitochondrial contents of other bird tissues for which only mtDNA copy number phenotypes (using the same qRT-PCR assay) exist (4). Building on our previous study on Cobb broilers (4) a revised breast muscle mitochondrial content of 2.05% (rather than the previously assumed 4%) suggests heart, drumstick and white fat have actual contents of 9.3%, 6.6% and 1.0%, respectively. These should be considered tentative predictions requiring independent validation by TEM, as the impact of tissue type on nuclear versus mitochondrial DNA extraction efficiency (and therefore the derived gDNA composition assayed by the molecular technology) remains unclear. Nevertheless, the predicted heart mitochondrial content value of 9.3% is dramatically lower than 10 differently sized mammals whose heart content has been typically measured to be between 24% (for larger mammals like dogs) − 37% (for smaller mammals like mice) (10).
The present finding of a relative reduction in breast muscle mitochondrial content in the modern more muscular and more feed efficient birds is entirely consistent with similar observations made in hyper-muscular, feed efficient breeds representing a number of mammalian production species. These include MSTN mutant cattle (11, 12), MSTN mutant sheep (13), Callipyge sheep (13–15), Large White pigs (16) and Yorkshire pigs (17). In those muscular, feed efficient breeds a muscle fibre composition shift favouring the low mitochondrial content Type IIB fibres has been repeatedly observed.
Indeed, an argument has been made that the gain in production efficiency is partially caused by the diminished tissue mitochondrial content (5) based on application of an economic design theory called symmorphosis (18). The logic runs as follows: it is inefficient to first construct (the cost of mitochondrial biogenesis) and then have to maintain (the cost of running the inner membrane proton gradient, inter alia) unnecessary bioenergetic capacity. The highly controlled environments we have created for intensively reared production animals, largely free of predation and other threats, open up the possibility of diminishing many aspects of physiological capacity in a quest for ever increasing gains in efficiency.
Nevertheless, the science relating aspects of the mitochondria (such as gene/protein expression) to feed efficiency is equivocal. For example, transcriptome analysis of the breast muscle of elite Cobb broilers differing in feed efficiency by 1.4 fold found that the most efficient birds expressed less myoglobin and less slow twitch contractile subunits (in line with expectation) but displayed higher expression of mRNA encoding mitochondrial proteins (contrary to expectation) (3). These basic findings were supported by an independent proteomics analysis performed on the same tissue samples suggesting they have been characterised correctly (19). One explanation is that abundance of mitochondrial proteins may not always have a simple relationship to mitochondrial content. After all, (20) found that more efficient Hereford x Angus steers had rumen tissue with lower mtDNA copy numbers but higher expression of mRNA encoding mitochondrial proteins. Data from multiple levels of biological organisation is probably desirable when interpreting mitochondrial function.
Where does a skeletal muscle mitochondrial content of just 2.05% sit in the broader context of life on earth? In fact, it is among the lowest measurements we could find for any muscle in any species. Intriguingly, it is at lower end of the estimations of 2.0 to 3.9% made for limb muscles of two wild cheetah, Acinonyx jubatus, the ultimate sprint adapted mammal (21). It is a staggering 17 fold lower than the 35.5% documented in the breast muscle of the highly athletic hummingbird, Selaphorus rufus (22). Indeed, the only lower estimates we could identify come from two large ruminants: the semitendinosus muscle in Zebu cattle, Bos indicus (1.1–1.3% ), and the longissimus dorsi muscle in Eland, Taurotragus oryx (1.4%) (23). Given tissue mitochondrial content scales negatively with body mass e.g. (5) such that smaller animals have higher contents, the extremely low breast muscle content observed here in ~ 3 kg birds is even more striking that when taken at face value. Moreover, given the breast muscle in these birds is 28% of total body mass, the implications for the birds’ overall bioenergetics are far greater than for the ruminant examples given.
One conclusion is certain. For a given muscle in every individual of every species, there must be a lower limit beyond which muscle mitochondrial content can no longer be further diminished. Beyond this metabolic ‘point of no return,’ sustainable aerobic ATP supply will be unable to satisfy basic cellular ATP demand and tissue damage will inevitably ensue. Even for low bioenergetic demand sedentary commercial broilers reared in highly controlled settings there are indications we may already be approaching that particular tipping point.
Concerning muscle pathologies have recently emerged, particularly noticeable in rapidly grown larger birds which we know are the very individuals possessing the lowest mitochondrial contents (4). These include ‘wooden breast’ (24) and ‘white striping,’ (25). These are conditions characterised by lipid infiltration, fibrosis and mitochondrial degeneration (26) in breast tissue - signs of a muscle system gone awry. Furthermore, evidence for hypoxia and lactic acidosis in afflicted birds (24, 25) indicates aerobic supply of ATP is not adequate to meet bioenergetic needs and that unsustainable use of anaerobic pathways of ATP generation have been resorted to. Other authors have indicated that a borderline inadequate capillary density in breast muscle is responsible (25), to which we would add the likely impact of a borderline inadequate mitochondrial content.
Taking this suite of comparative and production data together reinforces the notion that the modern broiler represents a remarkable and very extreme model of skeletal muscle function. We suggest that the broiler industry needs to be cognisant not to push the birds bioenergetic system too hard (i.e. too low) in the relentless quest for increasing feed efficiencies.
Limitations
This study is limited by the relatively small number of birds (n = 4 for TEM, n = 10 for qRT-PCR). The diet and behaviour of the birds prior to purchase was not quantitated in a controlled environment.