International Journal of Marine Science, 2025, Vol.15, No.3, 130-143 http://www.aquapublisher.com/index.php/ijms 135 On the phylogenetic tree, true horse mackerel is close to Korean horse mackerel (S. koreanus) and multi-line horse mackerel, while leopard horse mackerel and multi-line horse mackerel form a sister group relationship. It is worth noting that two genetic branches (named aff. guttatus 1 and aff. guttatus 2) appeared within the leopard horse mackerel, corresponding to the two major regional populations of the Pacific and Indian Oceans. This discovery suggests that the leopard mackerel may be further differentiated and may even represent two geographical subspecies or species. For the second largest lineage, the phylogenetic tree shows that the sharp-toothed mackerel (S. commerson) and the Queensland mackerel (S. queenslandicus) are clustered into one branch. This is consistent with the above speculation based on mitochondria, suggesting that there may be regional population differentiation of sharp-toothed mackerel frontiersin.org (Hashemi and Doustdar, 2022). 4.2 Discussion on the mechanism of speciation The reconstruction of the phylogenetic relationships of the Spanish mackerel provides a new perspective for understanding its speciation mechanism. According to the latest molecular evidence, geographic isolation (allopatric speciation) plays an important role in the evolution of species in this genus. The distribution areas of many Spanish mackerel species or lineages are separated by marine geographical barriers, which have led to independent evolution and genetic differentiation (Oliveira et al., 2021). This geographic isolation led to the differentiation of Spanish mackerel and Leopard Spanish mackerel. Similarly, the populations of Spanish mackerel in the Indian Ocean and the Western Pacific are blocked by the Southeast Asian Land Bridge and the Australian continent. Today, genetic data have detected the differentiation of its Pacific and Indian Ocean subpopulations. This suggests that the isolation of the two ocean environments and different ocean current systems may have contributed to the lineage differentiation within the Spanish mackerel. Different environmental pressures may lead to differences in physiology and behavior among populations, thereby promoting the formation of reproductive isolation. Although there has been no clear case of ecological differentiation speciation reported in Spanish mackerel, similar evidence has been found in other closely related fishes. For example, in some populations of Perciformes, differences in the number of individuals that develop under different water temperature conditions and misaligned reproduction times can lead to limited gene exchange and gradually lead to species differentiation. In recent years, extreme climate events (such as the El Niño phenomenon) may change the population connectivity pattern of Japanese mackerel and others. Model predictions show that if climate change continues, some mackerel populations (especially those distributed at the edge of the climate) may gradually become alienated from the core population, thus embarking on an independent evolutionary path (Lorenzen et al., 2021). Finally, it is worth mentioning the potential impact of human activities on the evolution of mackerel. The high-intensity fishing in modern fisheries may have a fishery-induced evolution (FIE) effect on some species, such as genetic responses such as early sexual maturity and smaller body size. Although such changes mainly occur within populations, they may also change the adaptation pattern of populations under long-term effects, thereby affecting the genetic differences between different populations. If some populations undergo obvious genetic trait changes due to fishing pressure, while other populations are not affected, then the reproductive barriers between the two may gradually increase, similar to "human-driven speciation (Pan et al., 2024)。 4.3 Phylogenetic differentiation on a time scale By calibrating the phylogenetic tree of the genus S. regalis using molecular clock methods, we can infer the time frame of its phylogenetic differentiation in geological history. Recent phylogenetic studies have used mitochondrial whole genome data to time the main lineages of the genus, revealing that the evolution of the genus S. regalis spanned a long period from the Paleocene to the Pleistocene (Oliveira et al., 2021). In the subsequent Eocene to Oligocene (about 56 million to 34 million years ago), the first major lineage differentiation occurred within the genus S. regalis. Clock analysis shows that the Old World group (mainly including species in today's Indo-West Pacific) and the New World group (American species, such as S. regalis, etc.) may have separated at the end of the Eocene to the beginning of the Oligocene.
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